PHYSIOLOGICAL AND MOLECULAR MECHANISMS OF LYMPHOCYTE HOMING

The lymphoid system is functionally compartmentalized in vivo into discrete primary, secondary, and tertiary lymphoid organs. Primary lymphoid tissues-the bone marrow and thymus-are responsible for the production of mature "virgin" lymphocytes. Secondary lymphoid tissues-lymph nodes, the spleen, and gut-associated lymphoid tissues-are specialized for the accumulation and presentation of antigen to both virgin and memory lymphocyte subsets. The remainder of the body's tissues may be considered "tertiary" lymphoid tissues, in that they normally contain only a few lymphoid elements, but in the setting of inflammation can be induced to recruit unique subsets of primarily memory lymphocytes. Each lymphoid tissue is further subdivided into discrete microenvironments, each characterized by a distinct complement of lymphocyte subsets and stromal cells. Lymphocyte homing comprises the physiologic processes by which lymphocytes seek out and localize to particular tissues and to specific microenvironments therein. Homing mechanisms play a major role in the maintenance of these specialized microenvironments and are critical for the dispersal and targeting of naive and memory lymphocyte populations that are required for effective immune surveillance. Here, we provide a brief overview of mechanisms thought to control the homing of lymphocyte populations in vivo, focusing in particular on the adhesive interactions involved in lymphocyte-endothelial cell recognition and in the selective extravasation of lymphocyte populations into secondary and tertiary lymphoid tissues.