The continuous traffic of lymphocytes through lymphoid organs is an essential component of the immune system: there are only few immunocompetent lymphocytes for a specific antigen and this traffic vastly increases their probability of meeting the antigen in the appropriate microenvironment around accessory cells and other lymphocyte subsets which are important for the initiation of effective immune responses. Lymphocyte traffic in general is not dependent on antigens. Sheep fetuses have normal lymphocyte recirculation through lymphoid tissues and into the thoracic duct, although they are secluded from external antigens by their particularly impervious placenta.

The entry into lymphoid organs through HEVs is regulated by a series of sequentially involved recep-tor-ligand interactions by several adhesion molecules. Certain models, e.g. the preferential route of 'memory' and 'naive' lymphocytes, have had to be revised recently. Much less is known about factors that may regulate transit within the lymphoid organs and the exit from the organ. Cytokines may play important roles in all these steps of transit through an organ. Little is known about the regulation of lymphocyte traffic to lymphoid organs without HEVs and to nonlymphoid organs such as the lung and liver.

See also: Adhesion molecules; Chemotaxis of lymphocytes; Endothelium; Fluorochrome labeling; High endothelial venules; Lifespan of immune cells and molecules; Lymphatic system; Proliferation, lymphocyte.

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