The secretory immunoglobulin system: regulation and biological significance. Focusing on human mammary glands.

Abstract

The surface of mucosal epithelia amounts to some 400 m2 in an adult individual. This extensive and generally quite vulnerable barrier is protected by numerous innate mechanisms that cooperate intimately with specific mucosal immunity. The main humoral mediators of this local immune system are secretory IgA (SIgA) and IgM (SIgM); the former class of antibodies constitutes the largest noninflammatory defense system of the body.1,2 Although the secretory antibody system is mainly directed against colonization of pathogens and penetration of “dangerous” antigens, it is also involved in immune exclusion of innocuous soluble proteins present in food (Figure 1). However, the latter type of antigen, as well as components of the indigenous microflora, generally induce poorly understood suppressive mechanisms collectively called oral tolerance when induced via the gut.3,4 This complex phenomenon of mucosally induced immunological downregulation apparently explains why most individuals show no adverse immune reactions to persistent contact with food proteins and the normal microbial flora.