Function of Antibodies
Antibodies are the primary defense against infectious pathogens or their products. Antibodies can be induced in the host actively by use of vaccines or acquired passively for conferring imme-diate protection against the pathogen. For example, hyperim-munized sera containing readymade antitoxins against toxins of tetanus, botulism, or diphtheria are given to neutralize the actions of these toxins immediately in the body. Also, hyperim-mune sera containing high titer of specific antibodies are given to inhibit attachment and replication of rabies and hepatitis A and B viruses early during the period of incubation. The func-tions of the antibodies can be summarized as follows:
Neutralization: By binding to the pathogen or foreign sub-stance, antibodies can block the binding of the pathogen with their targets. For example, antibodies to bacterial toxins can prevent the binding of the toxin to host cells, thereby ren-dering the toxin ineffective. Similarly, antibody binding to a virus or bacterial pathogen can block the attachment of the pathogen to its target cell, thereby preventing infection or colonization.
Opsonization: Antibody binding to a pathogen or foreignsubstance can opsonize the material and facilitate its uptake and destruction by phagocytic cells. The Fc region of other antibody interacts with Fc receptors on phagocytic cells, rendering the pathogen more readily phagocytosed.
Complement activation: Activation of the complementcascade by antibody can result in lysis of certain bacteria and viruses. In addition, some components of the complement cascade (e.g., C3b) opsonize pathogens and facilitate their uptake via complement receptors on phagocytic cells.