Melatonin mediates seasonal adjustments in immune function.

In addition to seasonal changes in reproductive function, seasonal changes in immune function are mediated by the pineal hormone, melatonin. Melatonin affects immune function both indirectly, acting through other hormones, and directly by acting on components of the immune system. Melatonin also affects tumorigenesis and tumor development. We hypothesize that many of the indirect effects of melatonin on immune function are mediated through glucocorticoids, and appear to be part of an integrated series of adaptations to manage energy. Direct effects of melatonin on immune function appear to be mediated by melatonin receptors on lymphatic tissue or on immune cells in circulation. Winter is energetically demanding and stressful; thermoregulatory demands typically increase when food availability decreases. Individuals would enjoy a survival advantage if seasonally recurring stressors could be anticipated and countered by bolstering immune function. To summarize, melatonin may be part of an integrative system to coordinate reproductive, immunologic and other physiological processes to cope successfully with energetic stressors during winter.