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 respons e to xenoantigen is modulated by daylength and dependent upon the pineal gl and. After injection of sheep erythrocytes (SRBC), serum immunoglobulin (Ig ) concentrations were 5-fold lower in hamsters in short versus long days. P inealectomy (Pinx) abolished the nocturnal melatonin rhythm, blocked short- day-mediated testis regression, and eliminated the short-day reduction in I g production after SRBC treatment. Antibody titers in response to SRBC were equivalently augmented in short-day Pinx and long-day sham hamsters. The r esults indicate that photoperiodic effects on T cell-dependent humoral immu nity are dependent upon the pineal gland. These findings raise the possibil ity that day length-associated changes in some immune system functions are mediated by the pineal melatonin rhythm.