The MALT is a separate compartment of the immune system from the peripheral (LN and spleen), body cavities, and the skin. Naive T cells exposed to antigen in one compartment become effector T cells that repeatedly home to that compartment.
T cells use specific adhesion molecules to recognize their compartment.
The gut is an entry site for food and for normal flora and pathogens. M cells "sample" antigens in the gut mucosa and transport them into the MALT for exposure to DC and lymphocytes.
Anatomy of the MALT
Activation and functions of [B2] B cells and Type a T cells (αβ CD4+ T cells, and αβ CD8+ T cells) is the same in the mucosal lymphoid tissues as in the LN and spleen, but IgA is the predominant antibody produced.
Approximately 50% of the T cells in the MALT are Type b T cells: γδ CD4-CD8- or γδ CD8αα T cells. αβ CD8αα T cells also are found in the MALT. These unusual T cells do not bind peptides on Class I and Class II MHC, but instead bind non-classical MHC molecules CD1, MIC-A and MIC-B. Their TCR repertoires are very restricted, and they do not undergo positive and negative selection in the thymus. One subset of γδ T cells uses the Vδ1 gene segment and also has the activating NK cell receptor that binds MIC-A and MIC-B, expressed on infected epithelial cells. The γδ T cells bind the infected epithelial cells and kill them.Some human T cells bearing the Vδ1 TCR also bind to CD1 that presents lipid and glycolipid antigens. These cells secrete IFNγ, which can polarize αβ T cells to make a Th1 response. The CD8αα molecule on T cells binds a nonclassical MHC expressed mostly by intestinal epithelial cells. It does not present peptides; instead it signals the T cells to be less cytotoxic but secrete more cytokines.
IgA is synthesized by plasma cells in the lamina propria and transported across the epithelial cells into the lumen. Low levels of IgA can also be secreted. They bind the poly Ig receptor on the basolateral side (inside) of the epithelial cell and cross the cytoplasm by transcytosis. On the lumenal side they are released containing a piece of the poly Ig receptor called secretory component that helps protect them from proteases. IgA antibodies to normal flora are produced by B1-B cells without the help of T cells and are not generally found in the serum unless the commensal bacteria become pathogenic and invade the bloodstream.
Enteric pathogens can cause disease by producing toxins in the lumen of the intestine. An example is Vibrio cholerae producing cholera toxin, which binds intestinal cells and causes them to excrete water. Many enteric pathogens can invade host tissues; the figure below shows some ways in which that can happen.
Most humans do not make immune responses to food antigens. Experiments with feeding mice antigens shows that oral exposure to antigens can result in anergy rather than immunity.
Review: Functions of Fc receptors (FcR).
