Immunoglobulin A

Abstract: Normal intestinal mucosa contains abundant immunoglobulin A (IgA)-secreting cells, which are generated from B cells in gut-associated lymphoid tissues (GALT). We show that dendritic cells (DC) from GALT induce T cell-independent expression of IgA and gut-homing receptors on B cells. GALT-DC-derived retinoic acid (RA) alone conferred gut tropism but could not promote IgA secretion. However, RA potently synergized with GALT-DC-derived interleukin-6 (IL-6) or IL-5 to induce IgA secretion. Consequently, mice deficient in the RA precursor vitamin A lacked IgA-secreting cells in the small intestine. Thus, GALT-DC shape mucosal immunity by modulating B cell migration and effector activity through synergistically acting mediators.

Abstract: Publisher Summary This chapter discusses the impact of new information concerning IgA physiology on the immune system. IgA should not be considered only as an isotype providing specific humoral protection of mucosal surfaces but as an integral component of the entire immune system. An unusual structural feature of human IgA is the heterogeneity of the molecular forms with characteristic distribution in various body fluids. Though most IgA in serum displays a typical four-polypeptide chain structure of the basic molecule with two Q and two light (L) chains, external secretions contain dimeric and tetrameric, disulfide-linked molecules associated with additional polypeptides-J (joining) chain and secretory component (SC). IgA-producing plasma cells are distributed in various lymphoid and nonlymphoid tissues and are particularly preponderant in the lamina propria of the gut; in salivary, lacrimal, and lactating mammary glands; and in the human bone marrow. IgA occurs in different body fluids in predominantly polymeric or monomeric (plasma, cerebrospinal fluid) forms with a characteristic distribution of IgAl and IgAz molecules. Under normal conditions, an absolute majority of IgA-containing cells in secretory glands and tissues also contain J chain whereas such cells in, for example, normal bone marrow does not. Staining with fluorochrome-labeled anti-J chain is enhanced by the pretreatment of alcohol-fixed tissue sections with acid urea, which leads to the exposure of masked antigenic determinants of intracellular J chain. Specialized lymphoid tissues associated with mucosal surfaces play an essential role in the induction and regulation of generalized immune responses in external secretions.