BIOLOGICAL CONSEQUENCES OF THE ANTIGEN-ANTIBODY REACTION
After binding to particulate antigens or after forming large molecular aggregates, antibod-ies unfold and may interact with Fc receptors on phagocytic cells. Such in-teraction is followed by ingestion by the phagocytic cell (phagocytosis). Substances that promote phagocytosis are known as opsonins.
The interaction of antigen-antibody complexes with phagocytic cells through their Fc re-ceptors results in the delivery of activating signals to the ingesting cell. When the Fc-re-ceptor–bearing cell is a phagocyte, the activation is usually associated with enhancement of its microbicidal activity. A less favorable outcome of phagocytic cell activation is an in-flammatory reaction, often trigged by spillage of the toxic mediators generated in the phagocytic cell after engagement of its Fc receptors. This outcome is more likely when the antigen-antibody complex is immobilized along a basement membrane or a cellular surface.
Another adverse reaction results from the engagement of Fc receptor–bound IgE on basophils and mast cells with their corresponding antigen. The result of this reaction is the release of the potent mediators that trigger an allergic reaction .
One of the most important consequences of antigen-antibody interactions is the activation (or “fixation”) of the complement system .
The activation sequence induced by antigen-antibody reactions is known as the “clas-sical” pathway. This pathway is initiated by the binding of C1q to the CH2 domain of the Fc region of IgG and equivalent regions of IgM. It must be noted that the complement-bind-ing sequences in IgG and IgM are usually not exposed in free antibody molecules, thus avoiding unnecessary and potentially deleterious activation of the complement system. The antigen-antibody interaction causes configurational changes in the antibody molecule, and the complement-binding regions become exposed.
The activation of C1q requires simultaneous interaction with two complement-bind-ing immunoglobulin domains. This means that when IgG antibodies are involved, rela-tively large concentrations are required, so that antibody molecules coat the antigen in very close apposition, allowing C1q to be fixed by IgG duplets. On the other hand, IgM molecules, by containing five closely spaced monomeric subunits, can fix complement at much lower concentrations. One IgM molecule bound by two subunits to a given antigen will constitute a complement-binding duplet.
After binding of C1q, a cascade reaction takes place, resulting in the successive activa-tion of eight additional complement components. Some of the components generated during complement activation are recognized by receptors on phagocytic cells and promote phago-cytosis. C3b is the complement fragment with greater opsonizing capacity. Phagocytic cells take up an antigen coated with opsonizing antibodies and C3b with maximal efficiency. Others, particularly the terminal complement components, induce cell lysis. These reactions have great biological significance and have been adapted to a variety of serological tests for diagnosis of infectious diseases.
The activation of the complement system may also have adverse effects, if it results in the destruction of host cells or if it promotes inflammation, which is beneficial with re-gard to the elimination of infectious organisms but always has the potential of causing tis-sue damage and becoming noxious to the host.
The binding of antibodies to bacteria, toxins, and viruses has protective effects because it prevents the interaction of the microbial agents or their products with the receptors that me- diate their infectiveness or toxic effects. As a consequence, the infectious agent or the toxin become harmless, or, in other words, are neutralized.