ABSTRACT
Lymphopenia, or decreased NK cell counts or activity after exercise are often referred to as immunosuppression. But unlike lymphopenia of HIV infection, which is an absolute loss of CD4 T cells, exercise or stress induces transient redistribution of lymphocytes which eventually will be recovered. Similarly NK cells may leave and come into the circulation in response to physical exercise and various stressors, both good and bad. Redistribution of CD4 T cells is mediated mainly by glucocorticoid in the blood stream, while NK cells are regulated by sympathetic outflow. The clinical significance of these fluctuation remains to be elucidated, but unlikely to lead to increased susceptibility to infection or cancer. They may rather be considered as a practical marker to know the activities of HPA axis or sympathetic activity.