The time of day of antigen encounter influences the magnitude of the immune response.

Studies in the rat and in man have shown that the time of day at which an antigen is encountered has an influence on the expression of any subsequent cell-mediated immunity, when the response is measured after a fixed interval. This suggests that immune processes are modulated by intrinsic biological rhythms. Experiments are now reported in which sensitized rats were ear-challenged with oxazolone and studied at intervals during the delayed hypersensitivity reaction. These show that differences in the expression of immunity due to the clock time of challenge became apparent at an early stage and were still present after the maximum response, assessed by the change in ear thickness. The experiments also show that the circadian variations in antigen responsiveness are present during the second recall of immunity and can be manipulated by altering the lighting regimen. The light-dark cycle is often a synchronizer for biological rhythms, although its precise role in the oxazolone system remains to be evaluated. This study demonstrates that phase reversal of the lighting regimen alters the proportions of lymphocytes present in rat blood at two clock times, one of which is the time of day at which the maximum immune response to oxazolone is initiated.