Mucosal immunity: The origin and migration patterns of cells in the secretory system

Abstract

It has been known for approximately 15 yr that secretory IgA (sIgA) is the predominant class of immunoglobulin present on certain mucous membranes which are exposed to the external environment.’ The IgA secreted onto the mucous membrane is, in large part, produced locally by plasma cells distributed in the lamina propria of the intestinal and respiratory tracts as well as other mucosal tissues.2 Recent information suggests that the transport of dimeric IgA and probably also of IgM is facilitated by complexing of these J chain-containing polymers with membranebound secretory component (SC) acting as a receptor. The binding of membrane SC with IgA and IgM may be a prerequisite for their pinocytotic transfer across the glandular epithelium. Consistent with this thesis is the demonstration of specific in vitro complexing of IgA and IgM with SC, the localization of SC (by immunofluorescence and electron microscopy) to the basal and lateral membranes of the epithelial cells, and the observation that the administration of anti-SC impairs the mucosal transport of IgA.3 Also, polymeric IgA and IgM will bind to the surface of fetal intestines at 16 wk gestation when SC first appears but not to sections taken earlier in gestation. SC may have an additional function in that when combined with IgA to form sIgA, an increased resistance to proteolytic degradation develops, compared to IgA dimers lacking SC or other classes of Ig.4T 5 Thus sIgA may be particularly suited (an evolutionary advantage?) to function in the “hostile” environment found at most mucosal surfaces.