Seasonal influence on mitogen and cyclosporin responses of peripheral blood lymphocytes

Percentages of unactivated lymphocytes, activated lymphoblasts and aberrant cells reflecting cytotoxicity were determined. Human lymphocytes demonstrated a significant decrease in response to PHA during the winter months, in comparison to the rest of the year. In contrast, the peripheral blood lymphocytes of rabbits housed under constant conditions did not demonstrate similar variations in response to PHA stimulation. The immunosuppressive action of cyclosporin A on this experimental model was unaffected by the observed seasonal variation in mitogen response in humans. These findings may guide research towards the identification of factors associated with the seasonality of the immune response and its potential influence on therapeutic interventions.