Role of photoperiod and the pineal gland in T cell-dependent humoral immune reactivity in the Siberian hamster

: The present study tested the hypothesis that antibody production in response to xenoantigen is modulated by daylength and dependent upon the pineal gland. After injection of sheep erythrocytes (SRBC), serum immunoglobulin (Ig) concentrations were 5‐fold lower in hamsters in short versus long days. Pinealectomy (Pinx) abolished the nocturnal melatonin rhythm, blocked short‐day‐mediated testis regression, and eliminated the short‐day reduction in Ig production after SRBC treatment. Antibody titers in response to SRBC were equivalently augmented in short‐day Pinx and long‐day sham hamsters. The results indicate that photoperiodic effects on T cell‐dependent humoral immunity are dependent upon the pineal gland. These findings raise the possibility that day length‐associated changes in some immune system functions are mediated by the pineal melatonin rhythm.