Lymphocyte recirculation, exercise, and immune responses.

Alterations in leukocyte concentrations in the blood are associated with exercise, stress, and other pathophysiological perturbations. The continuous migration and redistribution of cells of the recirculating lymphocyte pool between the blood and lymphatic systems can be influenced by a variety of physiological, immunological, and pathological processes. The phenotypic distribution of lymphocyte subsets is not the same in blood, afferent lymph, and efferent lymph, and cell-tracking experiments have shown that lymphocytes vary in their migratory properties. The most comprehensive physiological studies tracking these cells in vivo have been done in sheep. It has been shown that lymph-derived cells have different migratory capacities than blood-derived lymphocytes, that antigenic challenge of a single lymph node can first reduce the output of lymphocytes from the node and then markedly increase the recruitment from the blood and subsequently the output into efferent lymph. In most mammals, the blood pool of lymphocytes represents only about 1% of the total lymphocytes and only a small fraction of the recirculating lymphocyte pool. Therefore, testing the effects of exercise on lymphocyte recirculation by examining blood samples only requires considerable deduction and inference to interpret multicompartmental effects.