Affiliations: Faculty of Biology, Department of Animal Physiology, University of Warsaw, Warsaw, Poland
Note:  Correspondence to: Krystyna Skwarlo-Sonta, Faculty of Biology, Department of Physiology, University of Warsaw, Miecznikowa 1, 02-954 Warsaw, Poland. Tel.: +48 22 5541025; Fax: +48 22 5541203; E-mail: email@example.com
Abstract: Immune system activity in higher vertebrates (mammals and birds) varies over the year, mainly in correlation with the external light phase duration and intensity, as well as accompanying environmental cues. Light influences immunity mainly via retina-SCN-pineal gland-melatonin pathway. Diurnal rhythm of melatonin biosynthesis, adjusted to the changing external day length, acts as a message of darkness for the target cells/organs in the body, including the immune system. Some of melatonin effects are receptor mediated, while others are receptor independent. According to the trade-off hypothesis, seasonal changes in immunity are inversely related to reproduction, as both physiological functions are energetically costly while the resources are limited, especially in adverse environmental conditions. In the present review, the examples of the seasonal changes of immunity in mammalian and avian species are described and discussed in the context of the pineal gland biosynthetic activity. Current knowledge on the communication between the pineal gland/melatonin and immune system indicates that these relationships are complex and the precise mechanism(s) involved are still under investigation.