Localization of lymphocyte subpopulations in peripheral lymphoid organs: directed lymphocyte migration and segregation into specific microenvironments.

The distribution of lymphocytes in the peripheral lymphoid organs is controlled by recirculatory and microenvironmental factors. Specific interactions between recirculating lymphocytes and high endothelial venules in various lymphoid organs determine the presence and proportions of the various lymphoid sets and subsets in those organs. Separate endothelial determinants on peripheral node and Peyer's patch endothelium along with complementary lymphocyte receptors mediate this organ specificity. B and T cells also exhibit nonrandom organization within lymphoid tissues; after entry via high endothelial venules they segregate into their respective domains, which appear to be determined by distinct types of nonlymphoid stromal cells. Antigenic stimulation results in changes in lymphocyte phenotype as well as in the lymphoid microenvironment. The response to most complex antigens is the formation of germinal centers (GC) composed primarily of proliferating B cells; the phenotype of the few T cells therein is supportive of the GC as a site of B-T interaction. The phenotype of the B cells in GCs suggest a role for GCs in immunoglobulin class switching and the determination of subsequent homing specificity.