The humoral arm of the adaptive immune responses is particularly effective against extracellular microorganisms and their toxins. Antibodies bind to functionally critical antigenic sites on soluble toxins and to the surface antigens of extracellular microorganisms. Such binding effectively neutralizes toxins and microorganisms by preventing binding to host-cell surface molecules. Antibodies bound to bacteria are also able to activate a series of plasma proteins, known as complement, to produce molecules that are chemotactic for phagocytes, promote phagocytosis and can also directly destroy bacteria (Lambris et al., 1999).
Antibodies bind to bacteria by the amino-terminal antigen-binding sites, leaving the Fc component of the antibody exposed. Engagement of these exposed Fc fragments by surface Fc receptors on phagocytic cells induces phagocytosis and destruction of the coated bacterium; this process is known as opsonization. Macrophages and neutrophils possess IgM- and IgG-specific Fc receptors, while eosinophils possess IgE-specific Fc receptors. Phagocytes form part of the innate immune system and possess very limited antigen-specific receptors. Opsonizing antibodies enable phagocytes to recognize a wide range of antigens by effectively converting an antigen to an Fc segment that is easily recognized by phagocytes that are otherwise unable to engage and destroy the bacteria.
Antibodies are mainly directed against extracellular pathogens; however, they can be effective against virally infected cells that express viral antigens on their surfaces. These exposed viral antigens are bound by antigen-specific antibodies and the infected cell is destroyed by natural killer (NK) cells. NK cells are large granular lymphocytes, defined by the absence of surface immunoglobulin or T-cell receptors and the presence of Fc7 receptors. NK cells do not undergo clonal expansion; instead, they provide innate cytotoxic immune responses directed against virally infected cells, although they can interact with the adaptive immune response as outlined above (Fearon and Locksley, 1996).
Antibodies are highly effective against extracellular pathogens, but they have very limited potency against intracellular pathogens, such as viruses and certain bacteria. T-cells, however, are particularly effective against intracellular pathogens, because of their ability to identify infected cells and then mount and coordinate an effective cell mediated immune response.
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