StemCell Therapy

Most microorganisms encountered in daily life are repelled before they cause detectable signs and symptoms of disease. These potential pathogens, which include viruses, bacteria, fungi, protozoans, and worms, are quite diverse, and therefore a nonspecific defense system that diverts all types of this varied microscopic horde equally is quite useful to an organism. The innate immune system provides this kind of nonspecific protection through a number of defense mechanisms, which include physical barriers such as the skin, chemical barriers such as antimicrobial proteins that harm or destroy invaders, and cells that attack foreign cells and body cells harbouring infectious agents. The details of how these mechanisms operate to protect the body are described in the following sections.

The skin and the mucous membrane linings of the respiratory, gastrointestinal, and genitourinary tracts provide the first line of defense against invasion by microbes or parasites.

Human skin has a tough outer layer of cells that produce keratin. This layer of cells, which is constantly renewed from below, serves as a mechanical barrier to infection. In addition, glands in the skin secrete oily substances that include fatty acids, such as oleic acid, that can kill some bacteria; skin glands also secrete lysozyme, an enzyme (also present in tears and saliva) that can break down the outer wall of certain bacteria. Victims of severe burns often fall prey to infections from normally harmless bacteria, illustrating the importance of intact, healthy skin to a healthy immune system.

Like the outer layer of the skin but much softer, the mucous membrane linings of the respiratory, gastrointestinal, and genitourinary tracts provide a mechanical barrier of cells that are constantly being renewed. The lining of the respiratory tract has cells that secrete mucus (phlegm), which traps small particles. Other cells in the wall of the respiratory tract have small hairlike projections called cilia, which steadily beat in a sweeping movement that propels the mucus and any trapped particles up and out of the throat and nose. Also present in the mucus are protective antibodies, which are products of specific immunity. Cells in the lining of the gastrointestinal tract secrete mucus that, in addition to aiding the passage of food, can trap potentially harmful particles or prevent them from attaching to cells that make up the lining of the gut. Protective antibodies are secreted by cells underlying the gastrointestinal lining. Furthermore, the stomach lining secretes hydrochloric acid that is strong enough to kill many microbes.

Some microbes penetrate the bodys protective barriers and enter the internal tissues. There they encounter a variety of chemical substances that may prevent their growth. These substances include chemicals whose protective effects are incidental to their primary function in the body, chemicals whose principal function is to harm or destroy invaders, and chemicals produced by naturally occurring bacteria.

Some of the chemicals involved in normal body processes are not directly involved in defending the body against disease. Nevertheless, they do help repel invaders. For example, chemicals that inhibit the potentially damaging digestive enzymes released from body cells which have died in the natural course of events also can inhibit similar enzymes produced by bacteria, thereby limiting bacterial growth. Another substance that provides protection against microbes incidentally to its primary cellular role is the blood protein transferrin. The normal function of transferrin is to bind molecules of iron that are absorbed into the bloodstream through the gut and to deliver the iron to cells, which require the mineral to grow. The protective benefit transferrin confers results from the fact that bacteria, like cells, need free iron to grow. When bound to transferrin, however, iron is unavailable to the invading microbes, and their growth is stemmed.

A number of proteins contribute directly to the bodys nonspecific defense system by helping to destroy invading microorganisms. One group of such proteins is called complement because it works with other defense mechanisms of the body, complementing their efforts to eradicate invaders. Many microorganisms can activate complement in ways that do not involve specific immunity. Once activated, complement proteins work together to lyse, or break apart, harmful infectious organisms that do not have protective coats. Other microorganisms can evade these mechanisms but fall prey to scavenger cells, which engulf and destroy infectious agents, and to the mechanisms of the specific immune response. Complement cooperates with both nonspecific and specific defense systems.

Another group of proteins that provide protection are the interferons, which inhibit the replication of manybut not allviruses. Cells that have been infected with a virus produce interferon, which sends a signal to other cells of the body to resist viral growth. When first discovered in 1957, interferon was thought to be a single substance, but since then several types have been discovered, each produced by a different type of cell. Alpha interferon is produced by white blood cells other than lymphocytes, beta interferon by fibroblasts, and gamma interferon by natural killer cells and cytotoxic T lymphocytes (killer T cells). All interferons inhibit viral replication by interfering with the transcription of viral nucleic acid. Interferons exert additional inhibitory effects by regulating the extent to which lymphocytes and other cells express certain important molecules on their surface membranes.

In the small and large intestines the growth of invading bacteria can be inhibited by naturally gut-dwelling bacteria that do not cause disease. These gut-dwelling microorganisms secrete a variety of proteins that enhance their own survival by inhibiting the growth of the invading bacterial species.

If an infectious agent is not successfully repelled by the chemical and physical barriers described above, it will encounter cells whose function is to eliminate foreign substances that enter the body. These cells are the nonspecific effector cells of the innate immune response. They include scavenger cellsi.e., various cells that attack infectious agents directlyand natural killer cells, which attack cells of the body that harbour infectious organisms. Some of these cells destroy infectious agents by engulfing and destroying them through the process of phagocytosis, while other cells resort to alternative means. As is true of other components of innate immunity, these cells interact with components of acquired immunity to fight infection.

All higher animals and many lower ones have scavenger cellsprimarily leukocytes (white blood cells)that destroy infectious agents. Most vertebrates, including all birds and mammals, possess two main kinds of scavenger cells. Their importance was first recognized in 1884 by Russian biologist lie Metchnikoff, who named them microphages and macrophages, after Greek words meaning little eaters and big eaters.

Microphages are now called either granulocytes, because of the numerous chemical-containing granules found in their cytoplasm, or polymorphonuclear leukocytes, because of the oddly shaped nucleus these cells contain. Some granules contain digestive enzymes capable of breaking down proteins, while others contain bacteriocidal (bacteria-killing) proteins. There are three classes of granulocytesneutrophils, eosinophils, and basophilswhich are distinguished according to the shape of the nucleus and the way in which the granules in the cytoplasm are stained by dye. The differences in staining characteristics reflect differences in the chemical makeup of the granules. Neutrophils are the most common type of granulocyte, making up about 60 to 70 percent of all white blood cells. These granulocytes ingest and destroy microorganisms, especially bacteria. Less common are the eosinophils, which are particularly effective at damaging the cells that make up the cuticle (body wall) of larger parasites. Fewer still are the basophils, which release heparin (a substance that inhibits blood coagulation), histamine, and other substances that play a role in some allergic reactions (see immune system disorder: Allergies). Very similar in structure and function to basophils are the tissue cells called mast cells, which also contribute to immune responses.

Granulocytes, which have a life span of only a few days, are continuously produced from stem (i.e., precursor) cells in the bone marrow. They enter the bloodstream and circulate for a few hours, after which they leave the circulation and die. Granulocytes are mobile and are attracted to foreign materials by chemical signals, some of which are produced by the invading microorganisms themselves, others by damaged tissues, and still others by the interaction between microbes and proteins in the blood plasma. Some microorganisms produce toxins that poison granulocytes and thus escape phagocytosis; other microbes are indigestible and are not killed when ingested. By themselves, then, granulocytes are of limited effectiveness and require reinforcement by the mechanisms of specific immunity.

The other main type of scavenger cell is the macrophage, the mature form of the monocyte. Like granulocytes, monocytes are produced by stem cells in the bone marrow and circulate through the blood, though in lesser numbers. But, unlike granulocytes, monocytes undergo differentiation, becoming macrophages that settle in many tissues, especially the lymphoid tissues (e.g., spleen and lymph nodes) and the liver, which serve as filters for trapping microbes and other foreign particles that arrive through the blood or the lymph. Macrophages live longer than granulocytes and, although effective as scavengers, basically provide a different function. Compared with granulocytes, macrophages move relatively sluggishly. They are attracted by different stimuli and usually arrive at sites of invasion later than granulocytes. Macrophages recognize and ingest foreign particles by mechanisms that are basically similar to those of granulocytes, although the digestive process is slower and not as complete. This aspect is of great importance for the role that macrophages play in stimulating specific immune responsessomething in which granulocytes play no part.

Natural killer cells do not attack invading organisms directly but instead destroy the bodys own cells that have either become cancerous or been infected with a virus. NK cells were first recognized in 1975, when researchers observed cells in the blood and lymphoid tissues that were neither the scavengers described above nor ordinary lymphocytes but which nevertheless were capable of killing cells. Although similar in outward appearance to lymphocytes, NK cells contain granules that harbour cytotoxic chemicals.

NK cells recognize dividing cells by a mechanism that does not depend on specific immunity. They then bind to these dividing cells and insert their granules through the outer membrane and into the cytoplasm. This causes the dividing cells to leak and die.

NK cells are the third most abundant type of lymphocyte in the body (B and T lymphocytes being present in the greatest numbers). They develop from hematopoietic stem cells and mature in the bone marrow and the liver.

The body has a number of nonspecific methods of fighting infection that are called early induced responses. They include the acute-phase response and the inflammation response, which can eliminate infection or hold it in check until specific, acquired immune responses have time to develop. Nonspecific immune responses occur more rapidly than acquired immune responses do, but they do not provide lasting immunity to specific pathogens.

Nonadaptive immune responses rely on a number of chemical signals, collectively called cytokines, to carry out their effects. These cytokines include members of the family of proteins called interleukins, which induce fever and the acute-phase response, and tumour necrosis factor-alpha, which initiates the inflammatory response.

When the body is invaded by a pathogen, macrophages release the protein signals interleukin-1 (IL-1) and interleukin-6 (IL-6) to help fight the infection. One of their effects is to raise the temperature of the body, causing the fever that often accompanies infection. (The interleukins increase body temperature by acting on the temperature-regulating hypothalamus in the brain and by affecting energy mobilization by fat and muscle cells.) Fever is believed to be helpful in eliminating infections because most bacteria grow optimally at temperatures lower than normal body temperature. But fever is only part of the more general innate defense mechanism called the acute-phase response. In addition to raising body temperature, the interleukins stimulate liver cells to secrete increased amounts of several different proteins into the bloodstream. These proteins, collectively called acute-phase proteins, bind to bacteria and, by doing so, activate complement proteins that destroy the pathogen. The acute-phase proteins act similarly to antibodies but are more democraticthat is, they do not distinguish between pathogens as antibodies do but instead attack a wide range of microorganisms equally. Another effect the interleukins have is to increase the number of circulating neutrophils and eosinophils, which help fight infection.

Infection often results in tissue damage, which may trigger an inflammatory response. The signs of inflammation include pain, swelling, redness, and fever, which are induced by chemicals released by macrophages. These substances promote blood flow to the area, increase the permeability of capillaries, and induce coagulation. The increased blood flow is responsible for redness, and the leakiness of the capillaries allows cells and fluids to enter tissues, causing pain and swelling. These effects bring more phagocytic cells to the area to help eliminate the pathogens. The first cells to arrive, usually within an hour, are neutrophils and eosinophils, followed a few hours later by macrophages. Macrophages not only engulf pathogens but also help the healing process by disposing of cellular debris which accumulates from destroyed tissue cells and neutrophils that self-destruct after ingesting microorganisms. If infection persists, components of specific immunityantibodies and T cellsarrive at the site to fight the infection.

It has been known for centuries that persons who contract certain diseases and survive generally do not catch those illnesses again. Greek historian Thucydides recorded that, when the plague was raging in Athens during the 5th century bce, the sick and dying would have received no nursing at all had it not been for the devotion of those who had already recovered from the disease; it was known that no one ever caught the plague a second time. The same applies, with rare exceptions, to many other diseases, such as smallpox, chicken pox, measles, and mumps. Yet having had measles does not prevent a child from contracting chicken pox or vice versa. The protection acquired by experiencing one of these infections is specific to that infection; in other words, it is due to specific, acquired immunity, also called adaptive immunity.

There are other infectious conditions, such as the common cold, influenza, pneumonia, and diarrheal diseases, that can be caught again and again; these seem to contradict the notion of specific immunity. But the reason such illnesses can recur is that many different infectious agents produce similar symptoms (and thus the same disease). For example, more than 200 viruses can cause the cluster of symptoms known as the common cold. Consequently, even though infection with a particular agent does protect against reinfection by that same pathogen, it does not confer protection from other pathogens that have not been encountered.

Acquired immunity is dependent on the specialized white blood cells known as lymphocytes. This section describes the various ways in which lymphocytes operate to confer specific immunity. Although pioneer studies were begun in the late 19th century, most of the knowledge of specific immunity has been gained since the 1960s, and new insights are continually being obtained.

Lymphocytes are the cells responsible for the bodys ability to distinguish and react to an almost infinite number of different foreign substances, including those of which microbes are composed. Lymphocytes are mainly a dormant population, awaiting the appropriate signals to be stirred to action. The inactive lymphocytes are small, round cells filled largely by a nucleus. Although they have only a small amount of cytoplasm compared with other cells, each lymphocyte has sufficient cytoplasmic organelles (small functional units such as mitochondria, the endoplasmic reticulum, and a Golgi apparatus) to keep the cell alive. Lymphocytes move only sluggishly on their own, but they can travel swiftly around the body when carried along in the blood or lymph. At any one time an adult human has approximately 2 1012 lymphocytes, about 1 percent of which are in the bloodstream. The majority are concentrated in various tissues scattered throughout the body, particularly the bone marrow, spleen, thymus, lymph nodes, tonsils, and lining of the intestines, which make up the lymphatic system. Organs or tissues containing such concentrations of lymphocytes are described as lymphoid. The lymphocytes in lymphoid structures are free to move, although they are not lying loose; rather, they are confined within a delicate network of lymph capillaries located in connective tissues that channel the lymphocytes so that they come into contact with other cells, especially macrophages, that line the meshes of the network. This ensures that the lymphocytes interact with each other and with foreign materials trapped by the macrophages in an ordered manner.

Lymphocytes originate from stem cells in the bone marrow; these stem cells divide continuously, releasing immature lymphocytes into the bloodstream. Some of these cells travel to the thymus, where they multiply and differentiate into T lymphocytes, or T cells. The T stands for thymus-derived, referring to the fact that these cells mature in the thymus. Once they have left the thymus, T cells enter the bloodstream and circulate to and within the rest of the lymphoid organs, where they can multiply further in response to appropriate stimulation. About half of all lymphocytes are T cells.

Some lymphocytes remain in the bone marrow, where they differentiate and then pass directly to the lymphoid organs. They are termed B lymphocytes, or B cells, and they, like T cells, can mature and multiply further in the lymphoid organs when suitably stimulated. Although it is appropriate to refer to them as B cells in humans and other mammals, because they are bone-marrow derived, the B actually stands for the bursa of Fabricius, a lymphoid organ found only in birds, the organisms in which B cells were first discovered.

B and T cells both recognize and help eliminate foreign molecules (antigens), such as those that are part of invading organisms, but they do so in different ways. B cells secrete antibodies, proteins that bind to antigens. Since antibodies circulate through the humours (i.e., body fluids), the protection afforded by B cells is called humoral immunity. T cells, in contrast, do not produce antibodies but instead directly attack invaders. Because this second type of acquired immunity depends on the direct involvement of cells rather than antibodies, it is called cell-mediated immunity. T cells recognize only infectious agents that have entered into cells of the body, whereas B cells and antibodies interact with invaders that remain outside the bodys cells. These two types of specific, acquired immunity, however, are not as distinct as might be inferred from this description, since T cells also play a major role in regulating the function of B cells. In many cases an immune response involves both humoral and cell-mediated assaults on the foreign substance. Furthermore, both classes of lymphocytes can activate or enhance a variety of nonspecific immune responses.

Lymphocytes are distinguished from other cells by their capacity to recognize foreign molecules. Recognition is accomplished by means of receptor molecules. A receptor molecule is a special protein whose shape is complementary to a portion of a foreign molecule. This complementarity of shape allows the receptor and the foreign molecule to conform to each other in a fashion roughly analogous to the way a key fits into a lock.

Receptor molecules are either attached to the surface of the lymphocyte or secreted into fluids of the body. B and T lymphocytes both have receptor molecules on their cell surfaces, but only B cells manufacture and secrete large numbers of unattached receptor molecules, called antibodies. Antibodies correspond in structure to the receptor molecules on the surface of the B cell.

Any foreign materialusually of a complex nature and often a proteinthat binds specifically to a receptor molecule made by lymphocytes is called an antigen. Antigens include molecules found on invading microorganisms, such as viruses, bacteria, protozoans, and fungi, as well as molecules located on the surface of foreign substances, such as pollen, dust, or transplanted tissue. When an antigen binds to a receptor molecule, it may or may not evoke an immune response. Antigens that induce such a response are called immunogens. Thus, it can be said that all immunogens are antigens, but not all antigens are immunogens. For example, a simple chemical group that can combine with a lymphocyte receptor (i.e., is an antigen) but does not induce an immune response (i.e., is not an immunogen) is called a hapten. Although haptens cannot evoke an immune response by themselves, they can become immunogenic when joined to a larger, more complex molecule such as a protein, a feature that is useful in the study of immune responses.

Many antigens have a variety of distinct three-dimensional patterns on different areas of their surfaces. Each pattern is called an antigenic determinant, or epitope, and each epitope is capable of reacting with a different lymphocyte receptor. Complex antigens present an antigenic mosaic and can evoke responses from a variety of specific lymphocytes. Some antigenic determinants are better than others at effecting an immune response, presumably because a greater number of responsive lymphocytes are present. It is possible for two or more different substances to have an epitope in common. In these cases, immune components induced by one antigen are able to react with all other antigens carrying the same epitope. Such antigens are known as cross-reacting antigens.

T cells and B cells differ in the form of the antigen they recognize, and this affects which antigens they can detect. B cells bind to antigen on invaders that are found in circulation outside the cells of the body, while T cells detect only invaders that have somehow entered the cells of the body. Thus foreign materials that have been ingested by cells of the body or microorganisms such as viruses that penetrate cells and multiply within them are out of reach of antibodies but can be eliminated by T cells.

The specific immune system (in other words, the sum total of all the lymphocytes) can recognize virtually any complex molecule that nature or science has devised. This remarkable ability results from the trillions of different antigen receptors that are produced by the B and T lymphocytes. Each lymphocyte produces its own specific receptor, which is structurally organized so that it responds to a different antigen. After a cell encounters an antigen that it recognizes, it is stimulated to multiply, and the population of lymphocytes bearing that particular receptor increases.

How is it that the body has such an incredible diversity of receptors that are always ready to respond to invading molecules? To understand this, a quick review of genes and proteins will be helpful. Antigen receptor molecules are proteins, which are composed of a few polypeptide chains (i.e., chains of amino acids linked together by chemical bonds known as peptide bonds). The sequence in which the amino acids are assembled to form a particular polypeptide chain is specified by a discrete region of DNA, called a gene. But if every polypeptide region of every antigen receptor were encoded by a different gene, the human genome (all the genetic information encoded in the DNA that is carried on the chromosomes of cells) would need to devote trillions of genes to code just for these immune system proteins. Since the entire human genome contains approximately 25,000 genes, individuals cannot inherit a gene for each particular antigen receptor component. Instead, a mechanism exists that generates an enormous variety of receptors from a limited number of genes.

What is inherited is a pool of gene segments for each type of polypeptide chain. As each lymphocyte matures, these gene segments are pieced together to form one gene for each polypeptide that makes up a specific antigen receptor. This rearrangement of alternative gene segments occurs predominantly, though not entirely, at random, so that an enormous number of combinations can result. Additional diversity is generated from the imprecise recombination of gene segmentsa process called junctional diversificationthrough which the ends of the gene segments can be shortened or lengthened. The genetic rearrangement takes place at the stage when the lymphocytes generated from stem cells first become functional, so that each mature lymphocyte is able to make only one type of receptor. Thus, from a pool of only hundreds of genes, an unlimited variety of diverse antigen receptors can be created.

Still other mechanisms contribute to receptor diversity. Superimposed on the mechanism outlined in simplified terms above is another process, called somatic mutation. Mutation is the spontaneous occurrence of small changes in the DNA during the process of cell division. It is called somatic when it takes place in body cells (Greek soma means body) rather than in germ-line cells (eggs and sperm). Although somatic mutation can be a chance event in any body cell, it occurs regularly in the DNA that codes for antigen receptors in lymphocytes. Thus, when a lymphocyte is stimulated by an antigen to divide, new variants of its antigen receptor can be present on its descendant cells, and some of these variants may provide an even better fit for the antigen that was responsible for the original stimulation.

The antigen receptors on B lymphocytes are identical to the binding sites of antibodies that these lymphocytes manufacture once stimulated, except that the receptor molecules have an extra tail that penetrates the cell membrane and anchors them to the cell surface. Thus, a description of the structure and properties of antibodies, which are well studied, will suffice for both.

Antibodies belong to the class of proteins called globulins, so named for their globular structure. Collectively, antibodies are known as immunoglobulins (abbreviated Ig). All immunoglobulins have the same basic molecular structure, consisting of four polypeptide chains. Two of the chains, which are identical in any given immunoglobulin molecule, are heavy (H) chains; the other two are identical light (L) chains. The terms heavy and light simply mean larger and smaller. Each chain is manufactured separately and is encoded by different genes. The four chains are joined in the final immunoglobulin molecule to form a flexible Y shape, which is the simplest form an antibody can take.

At the tip of each arm of the Y-shaped molecule is an area called the antigen-binding, or antibody-combining, site, which is formed by a portion of the heavy and light chains. Every immunoglobulin molecule has at least two of these sites, which are identical to one another. The antigen-binding site is what allows the antibody to recognize a specific part of the antigen (the epitope, or antigenic determinant). If the shape of the epitope corresponds to the shape of the antigen-binding site, it can fit into the sitethat is, be recognized by the antibody. Chemical bonds called weak bonds then form to hold the antigen within the binding site.

The heavy and light chains that make up each arm of the antibody are composed of two regions, called constant (C) and variable (V). These regions are distinguished on the basis of amino acid similaritythat is, constant regions have essentially the same amino acid sequence in all antibody molecules of the same class (IgG, IgM, IgA, IgD, or IgE), but the amino acid sequences of the variable regions differ quite a lot from antibody to antibody. This makes sense, because the variable regions determine the unique shape of the antibody-binding site. The tail of the molecule, which does not bind to antigens, is composed entirely of the constant regions of heavy chains.

The variable and constant regions of both the light and the heavy chains are structurally folded into functional units called domains. Each light chain consists of one variable domain (VL) and one constant domain (CL). Each heavy chain has one variable domain (VH) and three or four constant domains (CH1, CH2, CH3, CH4). Those domains that make up the tail of the basic Y-shaped molecule (in other words, all the H-chain constant domains except CH1) are responsible for the special biological properties of immunoglobulinsexcept, of course, for the capacity to bind to a specific antigenic determinant. The tail of the antibody determines the fate of the antigen once it becomes bound to the antibody.

The hinge region of the antibody is a short stretch of amino acids on the heavy chain located between the chains CH1 and CH2 regions. It provides the molecule with flexibility, which is very useful in binding antigens. This flexibility can actually improve the efficiency with which an antigen binds to the antibody. It can also help in cross-linking antigens into a large lattice of antigen-antibody complexes, which are easily identified and destroyed by macrophages.

The term constant region is a bit misleading in that these segments are not identical in all immunoglobulins. Rather, they are basically similar among broad groups. All immunoglobulins that have the same basic kinds of constant domains in their H chains are said to belong to the same class. There are five main classesIgG, IgM, IgA, IgD, and IgEsome of which include a number of distinct subclasses. Each class has its own properties and functions determined by the structural variations of the H chains. In addition, there are two basic kinds of L chains, called lambda and kappa chains, either of which can be associated with any of the H chain classes, thereby increasing still further the enormous diversity of immunoglobulins.

IgG is the most common class of immunoglobulin. It is present in the largest amounts in blood and tissue fluids. Each IgG molecule consists of the basic four-chain immunoglobulin structuretwo identical H chains and two identical L chains (either kappa or lambda)and thus carries two identical antigen-binding sites. There are four subclasses of IgG, each with minor differences in its H chains but with distinct biological properties. IgG is the only class of immunoglobulin capable of crossing the placenta; consequently, it provides some degree of immune protection to the developing fetus. These molecules also are secreted into the mothers milk and, once they have been ingested by an infant, can be transported into the blood, where they confer immunity.

IgM is the first class of immunoglobulin made by B cells as they mature, and it is the form most commonly present as the antigen receptor on the B-cell surface. When IgM is secreted from the cells, five of the basic Y-shaped units become joined together to make a large pentamer molecule with 10 antigen-binding sites. This large antibody molecule is particularly effective at attaching to antigenic determinants present on the outer coats of bacteria. When this IgM attachment occurs, it causes microorganisms to agglutinate, or clump together.

IgA is the main class of antibody found in many body secretions, including tears, saliva, respiratory and intestinal secretions, and colostrum (the first milk produced by lactating mothers). Very little IgA is present in the serum. IgA is produced by B cells located in the mucous membranes of the body. Two molecules of IgA are joined together and associated with a special protein that enables the newly formed IgA molecule to be secreted across epithelial cells that line various ducts and organs. Although IgG is the most common class of immunoglobulin, more IgA is synthesized by the body daily than any other class of antibody. However, IgA is not as stable as IgG, and therefore it is present in lower amounts at any given time.

IgD molecules are present on the surface of most, but not all, B cells early in their development, but little IgD is ever released into the circulation. It is not clear what function IgD performs, though it may play a role in determining whether antigens activate the B cells.

IgE is made by a small proportion of B cells and is present in the blood in low concentrations. Each molecule of IgE consists of one four-chain unit and so has two antigen-binding sites, like the IgG molecule; however, each of its H chains has an extra constant domain (CH4), which confers on IgE the special property of binding to the surface of basophils and mast cells. When antigens bind to these attached IgE molecules, the cell is stimulated to release chemicals, such as histamines, that are involved in allergic reactions (see immune system disorder: Type I hypersensitivity). IgE antibodies also help protect against parasitic infections.

Most individuals have fairly constant amounts of immunoglobulin in their blood, which represent the balance between continuous breakdown of these proteins and their manufacture. There is about 4 times as much IgG (including its subclasses) as IgA, 10 to 15 times as much as IgM, 300 times as much as IgD, and 30,000 times as much as IgE.

Part of the normal production of immunoglobulin undoubtedly represents the response to antigenic stimulation that happens continually, but even animals raised in surroundings completely free from microbes and their products make substantial, though lesser, amounts of immunoglobulin. Much of the immunoglobulin therefore must represent the product of all the B cells that are, so to speak, ticking over even if not specifically stimulated. It is therefore not surprising that extremely sensitive methods can detect traces of antibodies that react with antigenic determinants to which an animal has never been exposed but for which cells with receptors are present.

All B cells have the potential to use any one of the constant-region classes to make up the immunoglobulin they secrete. As noted above, when first stimulated, most secrete IgM. Some continue to do so, but others later switch to producing IgG, IgA, or IgE. Memory B cells, which are specialized for responding to repeat infections by a given antigen, make IgG or IgA immediately. What determines the balance among the classes of antibodies is not fully understood. However, it is influenced by the nature and site of deposition of the antigen (for example, parasites tend to elicit IgE), and their production is clearly mediated by factors, called cytokines, which are released locally by T cells.

T-cell antigen receptors are found only on the cell membrane. For this reason, T-cell receptors were difficult to isolate in the laboratory and were not identified until 1983. T-cell receptors consist of two polypeptide chains. The most common type of receptor is called alpha-beta because it is composed of two different chains, one called alpha and the other beta. A less common type is the gamma-delta receptor, which contains a different set of chains, one gamma and one delta. A typical T cell may have as many as 20,000 receptor molecules on its membrane surface, all of either the alpha-beta or gamma-delta type.

The T-cell receptor molecule is embedded in the membrane of the cell, and a portion of the molecule extends away from the cell surface into the area surrounding the cell. The chains each contain two folded domains, one constant and one variable, an arrangement similar to that of the chains of antibody molecules. And, as is true of antibody structure, the variable domains of the chains form an antigen-binding site. However, the T-cell receptor has only one antigen-binding site, unlike the basic antibody molecule, which has two.

Many similarities exist between the structures of antibodies and those of T-cell receptors. Therefore, it is not surprising that the organization of genes that encode the T-cell receptor chains is similar to that of immunoglobulin genes. Similarities also exist between the mechanisms B cells use to generate antibody diversity and those used by T cells to create T-cell diversity. These commonalities suggest that both systems evolved from a more primitive and simpler recognition system.

Despite the structural similarities, the receptors on T cells function differently from those on B cells. The functional difference underlies the different roles played by B and T cells in the immune system. B cells secrete antibodies to antigens in blood and other body fluids, but T cells cannot bind to free-floating antigens. Instead they bind to fragments of foreign proteins that are displayed on the surface of body cells. Thus, once a virus succeeds in infecting a cell, it is removed from the reach of circulating antibodies only to become susceptible to the defense system of the T cell.

But how do fragments of a foreign substance come to be displayed on the surface of a body cell? First, the substance must enter the cell, which can happen through either phagocytosis or infection. Next, the invader is partially digested by the body cell, and one of its fragments is moved to the surface of the cell, where it becomes bound to a cell-surface protein. This cell-surface protein is the product of one of a group of molecules encoded by the genes of the major histocompatibility complex (MHC). In humans MHC proteins were first discovered on leukocytes (white blood cells) and therefore are often referred to as HLA (human leukocyte antigens). (For information on the genetic basis of the HLA, see human genetics.) There are two major types of MHC molecules: class I molecules, which are present on the surfaces of virtually all cells of the body that contain nucleithat is, most body cellsand class II molecules, which are restricted to the surfaces of most B cells and some T cells, macrophages, and macrophage-like cells.

Two main types of mature T cellscytotoxic T cells and helper T cellsare known. Some scientists hypothesize the existence of a third type of mature T cell called regulatory T cells. Some T cells recognize class I MHC molecules on the surface of cells; others bind to class II molecules. Cytotoxic T cells destroy body cells that pose a threat to the individualnamely, cancer cells and cells containing harmful microorganisms. Helper T cells do not directly kill other cells but instead help activate other white blood cells (lymphocytes and macrophages), primarily by secreting a variety of cytokines that mediate changes in other cells. The function of regulatory T cells is poorly understood. To carry out their roles, helper T cells recognize foreign antigens in association with class II MHC molecules on the surfaces of macrophages or B cells. Cytotoxic T cells and regulatory T cells generally recognize target cells bearing antigens associated with class I molecules. Because they recognize the same class of MHC molecule, cytotoxic and regulatory T cells are often grouped together; however, populations of both types of cells associated with class II molecules have been reported. Cytotoxic T cells can bind to virtually any cell in the body that has been invaded by a pathogen.

T cells have another receptor, or coreceptor, on their surface that binds to the MHC molecule and provides additional strength to the bond between the T cell and the target cell. Helper T cells display a coreceptor called CD4, which binds to class II MHC molecules, and cytotoxic T cells have on their surfaces the coreceptor CD8, which recognizes class I MHC molecules. These accessory receptors add strength to the bond between the T cell and the target cell.

The T-cell receptor is associated with a group of molecules called the CD3 complex, or simply CD3, which is also necessary for T-cell activation. These molecules are agents that help transduce, or convert, the extracellular binding of the antigen and receptor into internal cellular signals; thus, they are called signal transducers. Similar signal transducing molecules are associated with B-cell receptors.

When T-cell precursors leave the bone marrow on their way to mature in the thymus, they do not yet express receptors for antigens and thus are indifferent to stimulation by them. Within the thymus the T cells multiply many times as they pass through a meshwork of thymus cells. In the course of multiplication they acquire antigen receptors and differentiate into helper or cytotoxic T cells. As mentioned in the previous section, these cell types, similar in appearance, can be distinguished by their function and by the presence of the special surface proteins, CD4 and CD8. Most T cells that multiply in the thymus also die there. This seems wasteful until it is remembered that the random generation of different antigen receptors yields a large proportion of receptors that recognize self antigensi.e., molecules present on the bodys own constituentsand that mature lymphocytes with such receptors would attack the bodys own tissues.

Most such self-reactive T cells die before they leave the thymus, so that those T cells that do emerge are the ones capable of recognizing foreign antigens. These travel via the blood to the lymphoid tissues, where, if suitably stimulated, they can again multiply and take part in immune reactions. The generation of T cells in the thymus is an ongoing process in young animals. In humans large numbers of T cells are produced before birth, but production gradually slows down during adulthood and is much diminished in old age, by which time the thymus has become small and partly atrophied. Cell-mediated immunity persists throughout life, however, because some of the T cells that have emerged from the thymus continue to divide and function for a very long time.

B-cell precursors are continuously generated in the bone marrow throughout life, but, as with T-cell generation, the rate diminishes with age. Unless they are stimulated to mature, the majority of B cells also die, although those that have matured can survive for a long time in the lymphoid tissues. Consequently, there is a continuous supply of new B cells throughout life. Those with antigen receptors capable of recognizing self antigens tend to be eliminated, though less effectively than are self-reactive T cells. As a result, some self-reactive cells are always present in the B-cell population, along with the majority that recognize foreign antigens. The reason the self-reactive B cells normally do no harm is explained in the following section.

In its lifetime a lymphocyte may or may not come into contact with the antigen it is capable of recognizing, but if it does it can be activated to multiply into a large number of identical cells, called a clone. Each member of the clone carries the same antigen receptor and hence has the same antigen specificity as the original lymphocyte. The process, called clonal selection, is one of the fundamental concepts of immunology.

Two types of cells are produced by clonal selectioneffector cells and memory cells. Effector cells are the relatively short-lived activated cells that defend the body in an immune response. Effector B cells are called plasma cells and secrete antibodies, and activated T cells include cytotoxic T cells and helper T cells, which carry out cell-mediated responses.

The production of effector cells in response to first-time exposure to an antigen is called the primary immune response. Memory cells are also produced at this time, but they do not become active at this point. However, if the organism is reexposed to the same antigen that stimulated their formation, the body mounts a second immune response that is led by these long-lasting memory cells, which then give rise to another population of identical effector and memory cells. This secondary mechanism is known as immunological memory, and it is responsible for the lifetime immunities to diseases such as measles that arise from childhood exposure to the causative pathogen.

Helper T cells do not directly kill infected cells, as cytotoxic T cells do. Instead they help activate cytotoxic T cells and macrophages to attack infected cells, or they stimulate B cells to secrete antibodies. Helper T cells become activated by interacting with antigen-presenting cells, such as macrophages. Antigen-presenting cells ingest a microbe, partially degrade it, and export fragments of the microbei.e., antigensto the cell surface, where they are presented in association with class II MHC molecules. A receptor on the surface of the helper T cell then binds to the MHC-antigen complex. But this event alone does not activate the helper T cell. Another signal is required, and it is provided in one of two ways: either through stimulation by a cytokine or through a costimulatory reaction between the signaling protein, B7, found on the surface of the antigen-presenting cell, and the receptor protein, CD28, on the surface of the helper T cell. If the first signal and one of the second signals are received, the helper T cell becomes activated to proliferate and to stimulate the appropriate immune cell. If only the first signal is received, the T cell may be rendered anergicthat is, unable to respond to antigen.

A discussion of helper-T-cell activation is complicated by the fact that helper T cells are not a uniform group of cells but rather can be divided into two general subpopulationsTH1 and TH2 cellsthat have significantly different chemistry and function. These populations can be distinguished by the cytokines they secrete. TH1 cells primarily produce the cytokines gamma interferon, tumour necrosis factor-beta, and interleukin-2 (IL-2), while TH2 cells mainly synthesize the interleukins IL-4, IL-5, IL-6, IL-9, IL-10, and IL-13. The main role of the TH1 cells is to stimulate cell-mediated responses (those involving cytotoxic T cells and macrophages), while TH2 cells primarily assist in stimulating B cells to make antibodies.

Once the initial steps of activation have occurred, helper T cells synthesize other proteins, such as signaling proteins and the cell-surface receptors to which the signaling proteins bind. These signaling molecules play a critical role not only in activating the particular helper T cell but also in determining the ultimate functional role and final differentiation state of that cell. For example, the helper T cell produces and displays IL-2 receptors on its surface and also secretes IL-2 molecules, which bind to these receptors and stimulate the helper T cell to grow and divide.

The overall result of helper-T-cell activation is an increase in the number of helper T cells that recognize a specific foreign antigen, and several T-cell cytokines are produced. The cytokines have other consequences, one of which is that IL-2 allows cytotoxic or regulatory T cells that recognize the same antigen to become activated and to multiply. Cytotoxic T cells, in turn, can attack and kill other cells that express the foreign antigen in association with class I MHC molecules, whichas explained aboveare present on almost all cells. So, for example, cytotoxic T cells can attack target cells that express antigens made by viruses or bacteria growing within them. Regulatory T cells may be similar to cytotoxic T cells, but they are detected by their ability to suppress the action of B cells or even of helper T cells (perhaps by killing them). Regulatory T cells thus act to damp down the immune response and can sometimes predominate so as to suppress it completely.

A B cell becomes activated when its receptor recognizes an antigen and binds to it. In most cases, however, B-cell activation is dependent on a second factor mentioned abovestimulation by an activated helper T cell. Once a helper T cell has been activated by an antigen, it becomes capable of activating a B cell that has already encountered the same antigen. Activation is carried out through a cell-to-cell interaction that occurs between a protein called the CD40 ligand, which appears on the surface of the activated helper T cells, and the CD40 protein on the B-cell surface. The helper T cell also secretes cytokines, which can interact with the B cell and provide additional stimulation. Antigens that induce a response in this manner, which is the typical method of B-cell activation, are called T-dependent antigens.

Most antigens are T-dependent. Some, however, are able to stimulate B cells without the help of T cells. The T-independent antigens are usually large polymers with repeating, identical antigenic determinants. Such polymers often make up the outer coats and long, tail-like flagella of bacteria. Immunologists think that the enormous concentration of identical T-independent antigens creates a strong enough stimulus without requiring additional stimulation from helper T cells.

Interaction with antigens causes B cells to multiply into clones of immunoglobulin-secreting cells. Then the B cells are stimulated by various cytokines to develop into the antibody-producing cells called plasma cells. Each plasma cell can secrete several thousand molecules of immunoglobulin every minute and continue to do so for several days. A large amount of that particular antibody is released into the circulation. The initial burst of antibody production gradually decreases as the stimulus is removed (e.g., by recovery from infection), but some antibody continues to be present for several months afterward.

The process just described takes place among the circulating B lymphocytes. The B cells that are called memory cells, however, encounter antigen in the germinal centrescompartments in the lymphoid tissues where few T cells are presentand are activated in a different way. Memory cells, especially those with the most effective receptors, multiply extensively, but they do not secrete antibody. Instead, they remain in the tissues and the circulation for many months or even years. If, with the help of T cells, memory B cells encounter the activating antigen again, these B cells rapidly respond by dividing to form both activated cells that manufacture and release their specific antibody and another group of memory cells. The first group of memory cells behaves as though it remembers the initial contact with the antigen. So, for example, if the antigen is microbial and an individual is reinfected by the microbe, the memory cells trigger a rapid rise in the level of protective antibodies and thus prevent the associated illness from taking hold.

Many pathogenic microorganisms and toxins can be rendered harmless by the simple attachment of antibodies. For example, some harmful bacteria, such as those that cause diphtheria and tetanus, release toxins that poison essential body cells. Antibodies, especially IgG, that combine with such toxins neutralize them. Also susceptible to simple antibody attachment are the many infectious microbesincluding all viruses and some bacteria and protozoansthat live within the body cells. These pathogens bear special molecules that they use to attach themselves to the host cells so that they can penetrate and invade them. Antibodies can bind to these molecules to prevent invasion. Antibody attachment also can immobilize bacteria and protozoans that swim by means of whiplike flagella. In these instances antibodies protect simply by combining with the repeating protein units that make up these structures, although they do not kill or dispose of the microbes. The actual destruction of microbes involves phagocytosis by granulocytes and macrophages, and this is greatly facilitated by the participation of the complement system.

Complement is a term used to denote a group of more than 30 proteins that act in concert to enhance the actions of other defense mechanisms of the body. Complement proteins are produced by liver cells and, in many tissues, by macrophages. Most of these proteins circulate in the blood and other body fluids in an inactive form. They become activated in sequential fashion; once the first protein in the pathway is turned on, the following complement proteins are called into action, with each protein turning on the next one in line.

The action of complement is nonspecifici.e., complement proteins are not recognized by and do not interact with antigen-binding sites. In fact, complement proteins probably evolved before antibodies. Complement functions are similar among many species, and corresponding components from one species can carry out the same functions when introduced into another species. The complement system is ingenious in providing a way for antibodies, whatever their specificity, to produce the same biological effects when they combine with antigens.

Originally immunologists thought that the complement system was initiated only by antigen-antibody complexes, but later evidence showed that other substances, such as the surface components of a microorganism alone, could trigger complement activation. Thus, there are two complement activation pathways: the first one to be discovered, the classical pathway, which is initiated by antigen-antibody complexes; and the alternative pathway, which is triggered by other means, including invading pathogens or tumour cells. (The term alternative is something of a misnomer because this pathway almost certainly evolved before the classical pathway. The terminology reflects the order of discovery, not the evolutionary age of the pathways.) The classical and alternative pathways are composed of different proteins in the first part of their cascades, but eventually both pathways converge to activate the same complement components, which destroy and eliminate invading pathogens.

The classical complement pathway is activated most effectively by IgM and the most abundant of the immunoglobulins, IgG. But, for activation to occur, antibodies must be bound to antigens (the antigen-antibody complex mentioned above). Free antibodies do not activate complement. To initiate the cascade, the first complement protein in the pathway, C1, must interact with a bound immunoglobulin. Specifically, C1 interacts with the tail of the Y portion of the bound antibody moleculei.e., the nonspecific part of the antibody that does not bind antigen. Once bound to the antibody, C1 is cleaved, a process that activates C1 and allows it to split and activate the next complement component in the series. This process is repeated on the following proteins in the pathway until the complement protein C3the most abundant and biologically the most important component of the complement systemis activated. The classical and alternative complement pathways converge here, at the cleavage of the C3 molecule, which, once split, produces C3a and the large active form of C3, the fragment called C3b.

C3b carries out several functions:

It brings about lysis (bursting) of the target cell by activating subsequent steps in the cascade, leading to the formation of a ringlike structure called the membrane attack complex. This structure, which is composed of complement proteins C5 through C9, inserts itself into the membrane of the invading pathogen and creates a hole through which the cell contents leak out, killing the cell.

C3b can combine with another protein that converts more C3 protein to C3b.

C3b can initiate the alternative pathway of complement activation.

The small protein fragments that are released during the activation of complement are potent pharmacological agents that help promote an inflammatory response by causing mast cells and basophils to release histamine, which increases the permeability of blood vessels, and by attracting granulocytes and monocytes.

Thus, when a microbe penetrates the body, if antibodies reactive with its surface are already present (or if the microorganism activates complement without the help of antibodies, through the alternative complement pathway), the complete complement sequence may be activated and the microbe killed by damage to its outer membrane. This mechanism is effective only with bacteria that lack protective coats and with certain large viruses, but it is nevertheless important. Persons who lack C3 and thus cannot complete the later steps in the complement sequence are vulnerable to repeated bacterial infections.

Clearly such a biologically important chain of reactions could do more harm than good if its effects were to spread beyond the site of antigen invasion. Fortunately, the active intermediates at each stage in the complement sequence become rapidly inactivated or destroyed by inhibitors if they fail to initiate the next step. With rare exceptions, this confines the activation to the place in the body where it is needed.

Some cells that bear antigen-antibody complexes do not attract complement; their antibody molecules are far apart on the cell surface or are of a class that does not readily activate the complement system (e.g., IgA, IgD, and IgE). Other cells have outer membranes that are so tough or can be repaired so quickly that the cells are impermeable to activated complement. Still others are so large that phagocytes cannot ingest them. Such cells, however, can be attacked by killer cells present in the blood and lymphoid tissues. Killer cells, which may be either cytotoxic T cells or natural killer cells, have receptors that bind to the tail portion of the IgG antibody molecule (the part that does not bind to antigen). Once bound, killer cells insert a protein called perforin into the target cell, causing it to swell and burst. Killer cells do not harm bacteria, but they play a role in destroying body cells infected by viruses and some parasites.

The protection conferred by IgA antibodies, which are transported to the surface of mucous-membrane-lined passages, is somewhat different. Complement activation is not involved; there are no complement proteins in the lining of the gut or the respiratory tract. Here the available immune defense mechanism is primarily the action of IgA combining with microbes to prevent them from entering the cells of the lining. The bound microbes are then swept out of the body. IgA also appears to direct certain types of cell-mediated killing.

IgE antibodies also invoke unique mechanisms. As stated earlier, most IgE molecules are bound to special receptors on mast cells and basophils. When antigens bind to IgE antibodies on these cells, the interaction does not cause ingestion of the antigens but rather triggers the release of pharmacologically active chemical contents of the cells granules. The chemicals released cause a sudden increase in permeability of the local blood vessels, the adhesion and activation of platelets (blood cell fragments that trigger clotting), which release their own active agents, the contraction of smooth muscle in the gut or in the respiratory tubes, and the secretion of fluidsall of which tend to dislodge large multicellular parasites such as hookworms. Eosinophil granulocytes and IgE together are particularly effective at destroying parasites such as the flatworms that cause schistosomiasis. The eosinophils plaster themselves to the worms bound to IgE and release chemicals from their granules that break down the parasites tough protective skin. Therefore, IgE antibodiesalthough they can be a nuisance when they react with otherwise harmless antigensappear to have a special protective role against the larger parasites.

A newborn mammal has no opportunity to develop protective antibodies on its own, unless, as happens very rarely, it was infected while in the uterus. Yet it is born into an environment similar to its mothers, which contains all the potential microbial invaders to which she is exposed. Although the fetus possesses the components of innate immunity, it has few or none of its mothers lymphocytes. The placenta generally prevents the maternal lymphocytes from crossing into the uterus, where they would recognize the fetal tissues as foreign antigens and cause a reaction similar to the rejection of an incompatible organ transplant.

What is transferred across the placenta in many species is a fair sample of the mothers antibodies. How this happens depends on the structure of the placenta, which varies among species. In humans maternal IgG antibodiesbut not those of the other immunoglobulin classesare transported across the placenta into the fetal bloodstream throughout the second two-thirds of pregnancy. In many rodents a similar transfer occurs, but primarily across the yolk sac.

In horses and cattle, which have more layers of cells in their placentas, no antibodies are transferred during fetal life, and the newborn arrives into the world with no components of specific immunity. There is, however, a second mechanism that makes up for this deficiency. The early milk (colostrum) is very rich in antibodiesmainly IgA but also some IgM and IgGand during the first few days of life the newborn mammal can absorb these proteins intact from the digestive tract directly into the bloodstream. Drinking colostrum is therefore essential for newborn horses and cattle and required to a somewhat lesser extent by other mammals. The capacity of the digestive tract to absorb intact proteins must not last beyond one or two weeks, since once foods other than milk are ingested, the proteins and other antigens in them would also be absorbed intact and could act as immunogens to which the growing animal would become allergic (see immune system disorder: Allergies). IgA in milk is, however, rather resistant to digestion and can function within the gut even after intact absorption into the bloodstream has ended. Human colostrum is also rich in IgA, with the concentration highest immediately after birth.

After a newborn has received its supply of maternal antibodies, it is as fully protected as its mother. This means, of course, that if the mother has not developed immunity to a particular pathogen, the newborn will likewise be unprotected. For this reason, a physician may recommend that a prospective mother receive immunizations against tetanus and certain other disorders. (The active immunization of pregnant women against certain viral diseases, such as rubella [German measles], must be avoided, however, because the immunizing agent can cross the placenta and produce severe fetal complications.)

As important as the passively transferred maternal antibodies are, their effects are only temporary. The maternal antibodies in the blood become diluted as the animal grows; moreover, they gradually succumb to normal metabolic breakdown. Because the active development of acquired immunity is a slow and gradual process, young mammals actually become more susceptible to infection during their early stages of growth than they are immediately after birth.

Occasionally the transfer of maternal antibodies during fetal life can have harmful consequences. A well-known example of this is erythroblastosis fetalis, or hemolytic disease of the newborn, a disorder in which maternal antibodies destroy the childs red blood cells during late pregnancy and shortly after birth. The most severe form of erythroblastosis fetalis is Rh hemolytic disease, which develops when:

The mother is Rh-negative, which is to say her red blood cells lack the Rh factor.

Rh hemolytic disease can be prevented by giving the mother injections of anti-Rh antibody shortly after the birth of an Rh-positive child. This antibody destroys any Rh-positive fetal cells in the maternal circulation, thereby preventing the activation of the mothers immune system should she conceive another Rh-positive fetus.

In addition to their importance in cooperating with B cells that secrete specific antibodies, T cells have important, separate roles in protecting against antigens that have escaped or bypassed antibody defenses. Immunologists have long recognized that antibodies do not necessarily protect against viral infections, because many viruses can spread directly from cell to cell and thus avoid encountering antibodies in the bloodstream. It is also known that persons who fail to make antibodies are very susceptible to bacterial infections but are not unduly liable to viral infections. Protection in these cases results from cell-mediated immunity, which destroys and disposes of body cells in which viruses or other intracellular parasites (such as the bacteria that cause tuberculosis and leprosy) are actively growing, thus depriving microorganisms of their place to grow and exposing them to antibodies.

As discussed in the section Activation of T and B lymphocytes, cell-mediated immunity has two mechanisms. One involves activated helper T cells, which release cytokines. In particular, the gamma interferon produced by helper T cells greatly increases the ability of macrophages to kill ingested microbes; this can tip the balance against microbes that otherwise resist killing. Gamma interferon also stimulates natural killer cells. The second mechanism of cell-mediated immunity involves cytotoxic T cells. They attach themselves by their receptors to target cells whose surface expresses appropriate antigens (notably ones made by developing viruses) and damage the infected cells enough to kill them.

Cytotoxic T cells may kill infected cells in a number of ways. The mechanism of killing used by a given cytotoxic T cell depends mainly on a number of costimulatory signals. In short, cytotoxic T cells can kill their target cells either through the use of pore-forming molecules, such as perforins and various components of cytoplasmic granules, or by triggering a series of events with the target cell that activate a cell death program, a process called apoptosis. In general, the granular cytotoxic T cells tend to kill cells directly by releasing the potent contents of their cytotoxic granules at the site of cell-to-cell contact. This renders the cell membrane of the target cell permeable, which allows the cellular contents to leak out and the cell to die. The nongranular cytotoxic T cells often kill cells by inducing apoptosis, usually through the activation of a cell-surface protein called Fas. When a protein on the surface of the cytotoxic T cell interacts with the Fas protein on the target cell, Fas is activated and sends a signal to the nucleus of the target cell, thus initiating the cell death process. The target cell essentially commits suicide, thereby destroying the virus within the cell as well.

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Immune system - Britannica.com

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