Lymphoid cell populations dynamics and the mucosal immune response in humans

Investigations of the mucosal immune apparatus have for the most part concentrated on the function of animal and especially murine lymphocytes isolated from organized gut-associated lymphoreticular tissues (GALT) such as the Peyer’s patches. There is convincing evidence in several animal species that memory cells, and in particular antigen-sensitized B cells arising from GALT, might disseminate from these sites to remote secretory tissues. It is generally believed that ingested antigens, once absorbed in immunologically sufficient quantities by a specialised epithelium covering Peyer’s patches, can be processed and presented to underlying B and T lymphocytes. A fraction of locally activated B cells may leave the patches, transit through the mesenteric lymph nodes and thoracic duct, enter the circulation and while differentiating populate various secretory tissues including the gut, mammary and salivary glands, respiratory tract, where they further undergo terminal differentiation into plasma cells producing secretory antibodies, and especially in humans antibodies belonging predominantly to the IgA class (reviewed in [1]). These observations which have led to the concept of a “common or generalized mucosal immunological system” [2, 3] are especially important when considering strategies of vaccination against mucosal pathogens, since enteric delivery of immunogens is the most pratical and safe immunization route in humans and may soon constitute the most efficient mean to achieve immune protection against pathogens encountered at mucous membranes.