The large surface area of the intestinal tract is protected by secretory IgA produced locally by large numbers of plasma cells. Polymeric IgA (pIgA), composed of two to four IgA monomers and covalently linked J chain, is assembled in subepithelial plasma cells. pIgA is selectively transported by a secretory component-dependent mechanism into the intestinal lumen. The precursors of IgA-producing plasma cells display a unique migration pattern: IgA-committed and antigen-sensitized cells from the gut-associated lymphoid tissue populate remote mucosal surfaces and mature in these locales into IgA plasma cells. The process of IgA B-cells' differentiation is controlled at various steps by T cells and their soluble products, including interleukins and IgA-binding factors.