Organization of the mucosal immune system

In order to understand how mucosal immunity occurs, it is important to appreciate that foreign antigens, allergens and microbial pathogens arc encountered either through ingestion or by inhalation. The mammalian host has thus evolved organized second ary lymphoid tissues in these regions, which facilitate antigen uptake for processing and presentation for ultimate induction of mucosal immune responses. In mice, rats and rabbits (among others), a distinct BALT occurs which appears to take up and respond to intranasal/inhaled antigens; however in humans, BALT does not develop and the major inductive tissues appear to be the palatine tonsils and adenoids (nasopharyngeal tonsils), which together form a physical barrier of lymphoid tissues termed Waldey-er's ring. Thus, we now more frequently describe tonsils and adenoids as nasal-associated lymphoid tissue (NALT) and assume that NALT are major inductive sites for intranasal/inhaled antigens. Likewise, we assume that GALT and especially the Pey-er's patches are major inductive sites in the gastrointestinal tract so that collectively, MALT consists of NALT and GALT in humans and of NALT, BALT and GALT in experimental mammalian systems. These MALT inductive sites contain B and T lymphocytes which respond, in the presence of appropriate APCs, to the encountered antigen by developing into effector and memory B and T cells. These antigen-specific B and T cell populations then migrate from the inductive environment via lymphatic drainage, circulate through the bloodstream and home to mucosal effector regions. Thus, mucosal effector sites include the more diffuse tissues where antigen-spe-cific T and B lymphocytes ultimately reside and perform their respective functions (i.e. cytotoxicity, cytokine production or antibody synthesis) to protect mucosal surfaces.

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