[Secretory IgA and the antibacterial defenses of the respiratory tract (author's transl)].

The secretory IgA (IgA S) in bronchial mucus is produced by cells which are derived, at least in part, from the BALT (Bronchus Associated Lymphoid Tissue). The physico-chemical characteristics of IgA S (the amino-acid structure of the alpha chain, the secretory component and the J chain) explain three properties of this antibody which enable it to play its part as the first line of defense on the surface of the mucus, namely its resistance to proteolysis, its adherence and its ability to penetrate mucus. The role of IgA S in the antibacterial defense of the respiratory tract has been in doubt for some time because it does not follow the usual processes of antibacterial action, having neither opsonising nor lytic activity. It is thought at present that IgA S exerts its action in an individual way, more directly and peculiar to the secretory immune-apparatus. It acts by 1) causing agglutination of bacteria and by inhibiting their adsorption on the mucous membranes, 2) inhibiting bacterial reproduction and 3) by inactivating bacterial toxins. The combination of these three processes produces a surface defense system which inhibits bacterial penetration of the mucus. Despite the uncertainties which still exist, it is essential that the fundamental immunological data should be included in the complex picture of the mechanisms of defense of the respiratory tract.