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 above—stimulation 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 centres—compartments in the lymphoid tissues where few T cells are present—and 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 microbes—including all viruses and some bacteria and protozoans—that 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 nonspecific—i.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 molecule—i.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 C3—the most abundant and biologically the most important component of the complement system—is 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:
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 fluids—all 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 parasite’s tough protective skin. Therefore, IgE antibodies—although they can be a nuisance when they react with otherwise harmless antigens—appear to have a special protective role against the larger parasites.