THE INFLUENCE OF SEASON, PHOTOPERIOD, AND PINEAL MELATONIN ON IMMUNE FUNCTION

In addition to the well-documented seasonal cycles of mating and birth , there are also significant seasonal cycles of illness and death amon g many animal populations. Challenging winter conditions (i.e., low am bient temperature and decreased food availability) can directly induce death via hypothermia, starvation, or shock. Coping with these challe nges can also indirectly increase morbidity and mortality by increasin g glucocorticoid secretion, which can compromise immune function. Many environmental challenges are recurrent and thus predictable; animals could enhance survival, and presumably increase fitness, if they could anticipate immunologically challenging conditions in order to cope wi th these seasonal threats to health. The annual cycle of changing phot operiod provides an accurate indicator of time of year and thus allows immunological adjustments prior to the deterioration of conditions. P ineal melatonin codes day length information. Short day lengths enhanc e several aspects of immune function in laboratory studies, and melato nin appears to mediate many of the enhanced immunological effects of p hotoperiod. Generally, field studies report compromised immune functio n during the short days of autumn and winter. The conflict between lab oratory and field data is addressed with a multifactor approach. The e vidence for seasonal fluctuations in lymphatic tissue size and structu re, as well as immune function and disease processes, is reviewed. The role of pineal melatonin and the hormones regulated by melatonin is d iscussed from an evolutionary and adaptive functional perspective. Fin ally, the clinical significance of seasonal fluctuations in immune fun ction is presented. Taken together, it appears that seasonal fluctuati ons in immune parameters, mediated by melatonin, could have profound e ffects on the etiology and progression of diseases in humans and nonhu man animals. An adaptive functional perspective is critical to gain in sights into the interaction among melatonin, immune function, and dise ase processes.