Circadian rhythmicity of human plasma cortisol and PHA-induced lymphocyte transformation

In two experiments, lymphocytes from six healthy subjects were sampled every 4 hr, counted, and cultured with PHA. Plasma cortisol in the same blood samples was measured and showed an expected circadian variation in level. A circadian rhythm in lymphocyte transformability to PHA was found to coincide directly with the cortisol level but varied inversely with lymphocyte numbers. The relationship between cortisol and lymphocyte transformability could be fortuitous or might indicate a role for cortisol in controlling lymphocyte function in vivo.