Pathways of Cell Recruitment to Mucosal Surfaces
The mucosal immune system, consisting of mucosal tissues associated with the lacrimal, salivary, gastrointestinal, respiratory, and urogenital tracts and lactating breasts, qualitatively makes up the majority of the lymphoid tissue in the body. The gastrointestinal mucosal immune system has a number of features: it contains specialized structures, such as the Peyer’s patches (PPs), where primary immune responses are though to be initiated, followed by patterns of specific recirculation of lymphocyte to mucosal tissues. Subsets of lymphoid cells, primarily IgA-positive B cells and memory T cells predominate at mucosal surfaces, and the predominant mucosal immunoglobulin, secretory IgA, is particularly well adapted to host defenses at mucosal surfaces. This combination of factors in the gastrointestinal immune system thus serves two specific purposes—to protect the host from harmful pathogens while at the same time being tolerant of ubiquitous dietary antigens and microbial flora. In this chapter we emphasize the mechanisms by which leukocytes (primarily lymphocytes) acquire a phenotype to selectively localize from the circulation to mucosal sites.