The secretory IgA system.

The secretory IgA system is common to all mucosal membranes and is presumably of great importance for their defense. In addition to the secretory IgA antibodies produced in a mucosa in response to a local antigenic stimulus there is a spread of this type of IgA response via committed lymphocytes. They originate from central lymphoid organs in the intestinal (Peyer's patches) and bronchial mucosa (bronchus-associated lymphoid tissue, BALT) which they leave after antigenic exposure. They migrate, or "home", to exocrine glands such as the lacrimal, salivary, mammary and prostatic glands and mucosal membranes of the respiratory, gastrointestinal and genito-urinary tract. Almost half of all lymphocytes may be involved in the production of IgA antibodies. The secretory IgA antibodies are the dominating immunoglobulins in exocrine secretions on mucous membranes. They function primarily by preventing contact between the microbe and the host tissue most commonly attacked in infections, the mucous membrane. The fact that breast-feeding protects the infant against intestinal infections is one good example of the clinical significance of secretory IgA antibodies. This mode of protection can be enhanced by vaccination.