ABSTRACT
The mucosal immune system has long been recognized as the first line of defense against microbial invaders. The aerodigestive and the urogenital tracts as well as the eye conjunctiva and the ducts of all exocrine glands are all covered by mucous membranes with their highly specialized immune system. As described by Cwerkinsky et al. [I], this consists of an integrated and communicating network of lymphoid cells which works in concert with innate host factors to promote host defense. Major mucosal effector immune mechanisms include secretory antibodies, largely of immunoglobulin A (IgA) isotype, cytotoxic T-cells, and cytokines, chemokines, and their receptors. In a healthy human adult, this local immune system contributes almost 80% of all immunocytes [I]. These cells are accumulated in or transit between various mucosal organs and glands and together form the mucosa-associated lymphoid tissue (MALT), the largest mammalian lymphoid organ system. MALT comprises anatomically defined lymphoid microcompartments, such as the Peyer's patches in the small intestine, the appendix and solitary follicles in the large intestine and in the rectum, the nasal mucosa and the tonsils at the entrance of the aerodigestive tract, which serve as the principal mucosal inductive site where immune responses are being initiated. MALT contains diffuse accumulations of large numbers of lymphoid cells that do not associate into apparently organized structures. These cells are either distributed in the lamina propria or interspersed among epithelial cells in mucosal tissues and glands, giving rise to the mucosal effector sites where immune responses are being induced and/or expressed. The gut mucosa is particularly well endowed with such diffuse lymphoid tissues. More important for the elicitation of an immune response is the fact that immunization at certain inductive sites may give rise to a humoral immune response preferentially manifested at certain effector sites. Thus, a given inductive site may serve as a preferential but not exclusive source of precursor cells for certain mucosal tissues. MALT mainly functions to protect the mucous membranes against colonization by potentially dangerous microbes as well as to prevent the development of harmful responses to these antigens (Table I).
