Defence of mucous membranes by antibodies, receptor analogues and non-specific host factors.

Most infections reach man via the mucosal membranes, and more than half of the lymphoid system is found in connection with mucosae. The major antibodies found on mucous membranes are secretory IgA, which function primarily by binding microorganisms and thereby preventing their contact with the host tissues. The optimal mode of immunization to obtain a secretory IgA response is not well defined. Repeated mucosal exposure with antigen may result in oral tolerance, with decreasing circulating antibodies but a remaining secretory IgA response. The secretory IgA response is usually short-lived and can be difficult to boost. IgM as well as IgG antibodies may add to host defence at the mucosal level, but when engaged, they usually induce inflammation in host tissues. Analogues to bacterial receptors on mucosal epithelium may be present in exocrine secretions such as human milk. During an attack on the host, it is possible that such receptor analogues may aid in the prevention of attachment of bacteria to mucous membranes used as an initial site. A number of non-specific host factors support mucosal defence. One of them is lactoferrin. Lactoferrin deficiency seems to result in recurrent bacterial infections, suggesting its importance in normal host defence.