Summary and Afterthoughts on Session 3: Homing of Cells and Inductive Effects of Microenvironment and of Humoral Factors

Abstract

This session was characterized by considerations of structural and fluid phase microenvironments in the antigen independent development and traffic patterns of T and B lymphocytes, as well as alterations in traffic and location of these cells following antigenic stimulation. Maria de Sousa introduced the session by reviewing concepts of the specific recognition between circulating lymphocytes and specialized endothelial cells (via post capillary venules in lymph nodes and Peyer’s patches; marginal sinuses in the spleen) which allows entry of these lymphocytes (but not other blood borne cells) into lymphoid tissue parenchyma. She demonstrated that the failure to observe positive lymphoid tissue entry in some experimental situations need not imply the lack of proper recognition between lymphocytes and specialized endothelium; rather, augmented trapping by nonlymphoid reticuloendothelial tissues may remove these cells from the available pool. She also reviewed the evidence that specific homing to T and B cell regions of lymphoid tissues occurs, and this implies a second discriminative recognition process. Later in the session ,Weissman reviewed work by Gutman and Friedberg in his lab that both T⁺ and Ig⁺ lymphocytes enter lymphoid tissues via these specialized vessels in a random fashion.