Ontogeny of the Local Immune System

Abstract

The classic studies of Tomasi and his colleagues' in early 1960s demonstrated that the predominant class of immunoglobulin in various external fluids was of IgA class, although it is a relatively minor immunoglobulin in serum. These observations represent the foundations of the earlier concept of the secretory immune system. Subsequent studies conducted during the last decade have made it clear that in addition to IgA, other immunoglobulin (IgM, IgE, IgG), components of T-cell-mediated immunity and other nonimmunologic factors are also present in significant amounts in the external mucosal secretions.2,3 Thus, the current concept of the local (secretory) immune system includes the secretory IgA, other immunoglobulins and cell-mediated immune reactivity. The local immune system is referred to the complex mechanisms of specific immunity existing on the external mucosal surfaces and which appear to operate somewhat independent of the immune function in other systemic sites in man and most other mammalian species. The local immune system includes the bronchus associated lymphoid tissue (BALT), gut associated lymphoid tissue (GALT), and the immunocompetent cell or organized lymphoid follicles in the conjunctiva, salivary glands, and upper respiratory tract, genitourinary tract and mammary glands.4-6 A number of publications3,4 have extensively reviewed the development and function of the secretory immune system. In this report, an attempt will be made to provide an overview of the more recent developments in the mechanisms of local immunity. SECRETORY IMMUNOGLOBULINS Structure of Secretory IgA The bulk of the IgA immunoglobulin in external secretions exist as a complex molecule which consist of three immunologically distinct components: (1) a dimer of two 7 S IgA molecules, (2) the J (joining) chain, and (3) the secretory component (SC).