Seasonal changes in immune function

Author: R.J. Nelson and G.E. Demas
Abstract: Winter is energetically demanding. Physiological and behavioural adaptations have evolved among nontropical animals to cope with winter because thermoregulatory demands increase when food availability decreases. Seasonal breeding is central within the suite of winter adaptations among small animals. Presumably, reproductive inhibition during winter conserves energy at a time when the odds of producing viable young are low. In addition to the well-studied seasonal cycles of mating and birth, there are also significant seasonal cycles of illness and death among many populations of mammas and birds in the field. Challenging winter conditions, such as low ambient temperatures and decreased food availability, can directly induce death via hypothermia, starvation or shock. In some cases, survival in demanding winter conditions puts individuals under great physiological stress, defined here as an adaptive process that results in elevated blood levels of glucocorticoids. The stress of coping with energetically demanding conditions can also indirectly cause illness and death by compromising immune function. Presumably, the increased blood concentrations of adrenocortical steroids in response to winter stressors compromise immune function and accelerate catabolic mechanisms in the field, although the physiological effects of elevated glucocorticoids induced by artificial stressors have been investigated primarily in the laboratory. However, recurrent environmental stressors could reduce survival if they evoke persistent glucocorticoid secretion. The working hypothesis of this article is that mechanisms have evolved in some animals to combat season stress-induced immunocompromise as a temporal adaptation to promote survival. Furthermore, we hypothesise that mechanisms have evolved that allow individuals to anticipate periods of immunologically challenging conditions. A review of the effects of photoperiod on immune system function in laboratory studies reveals that exposure to short day lengths enhances immune function in every species examined. Short day exposure in small mammals cause reproductive inhibition and concomitant reduction in plasma levels of prolactin and steroid hormones as well as alterations in the temporal pattern of pineal melatonin secretion. Conclusion: day length appears to affect immune function in many species, including animals that typically do not exhibit reproductive responsiveness to day length.
Keywords: inhibition, reproduction, immune system function, seasonal, short days, mammals, animals, photoperio
Publication: Quarterly Review of Biology
Issue: Volume 71 Number 4
Dated: December 1966
Pages: 511-548

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