(A) IgA # Mucosa-associated lymphoid tissues lining the gut are known as gut-associated lymphoid tissue or GALT. The tonsils and adenoids form a ring, known as Waldeyer's ring, at the back of the mouth at the entrance of the gut and airways.> The mucosal immune system contains a distinctive repertoire of lymphocytes> Secretory IgA is the antibody isotype associated with the mucosal immune system> Most antigens presented to the mucosal immune system induce tolerance.> The mucosal immune system can mount an immune response to the normal bacterial flora of the gut> Polymeric IgA and IgM are synthesized throughout the gut by plasma cells located in the lamina propria and are transported into the gut by immature epithelial cells located at the base of the intestinal crypts.> Secreted IgA and IgM bind to the mucus layer overlying the gut epithelium where they can bind to and neutralize gut pathogens and their toxic products