[Effector mechanisms of IgA].

Like all immunoglobulins (Ig), IgA has a double function: recognition of the antigen, situated in the Fab alpha fragments, and effector functions which allow elimination of the antigen (Ag), carried by the Fc alpha fragment. Secretory IgA ( IgAs ) is the principal Ig of external secretions and mucosae and has a different structure and composition from serum IgA. Its external function of protection against various forms of bacterial and viral aggression has been well established. Its general mechanism is "the immune exclusion of antigens" i.e. prevention of the penetration of the Ag into the organism by confining them to external secretions followed by elimination. The elimination of bacteria is facilitated by the immobilization and agglutination by IgA. Sometimes, with the aid of lysozyme, IgAs can be bacteriolytic. Antibacterial IgAs have a bacteriostatic function in synergy with lactoferrin and/or transferrin; they even reduce the bacterial production of siderophores. IgAs can inhibit bacterial adhesion to epithelial cells and can increase their adherence to mucus. The neutralization of viruses by IgA is due to inhibition of the first stage of infection, attachment and intracellular penetration of the virus. The same mechanism is involved in the neutralization of bacterial toxins. IgAs also decrease intestinal absorption of foreign proteins in the diet. It has been reported that antibacterial IgAs can cause certain bacteria to lose a plasmid which determines their infectivity. IgAs are able to protect themselves by neutralizing IgA1-proteases secreted by certain bacteria found on mucosal surfaces. Plasma IgA has a limited internal action compared to IgM or IgG. It is generally accepted that IgA barely activates complement (C) by the classical pathway and minimally opsonizes Ag for mono- and polymorphonuclear phagocytes. Antibacterial IgA are not bactericidal in the presence of complement and do not facilitate the phagocytosis of the bacteria to which they are attached. Certain unfavourable effector functions have even been described, such as a specific inhibition of complement fixation and bacterial lysis by IgM and IgG. Some IgA may non-specifically inhibit neutrophil chemotaxis as well as their bactericidal and phagocytic activities. It is difficult to believe that IgA, the second Ig (in quantity) in human serum, is simply useless or even harmful.(ABSTRACT TRUNCATED AT 400 WORDS)