Sunlight Effects on Immune System: Is There Something Else in addition to UV-Induced Immunosuppression?

Abstract

Sunlight, composed of different types of radiation, including ultraviolet wavelengths, is an essential source of light and warmth for life on earth but has strong negative effects on human health, such as promoting the malignant transformation of skin cells and suppressing the ability of the human immune system to efficiently detect and attack malignant cells. UV-induced immunosuppression has been extensively studied since it was first described by Dr. Kripke and Dr. Fisher in the late 1970s. However, skin exposure to sunlight has not only this and other unfavorable effects, for example, mutagenesis and carcinogenesis, but also a positive one: the induction of Vitamin D synthesis, which performs several roles within the immune system in addition to favoring bone homeostasis. The impact of low levels of UV exposure on the immune system has not been fully reported yet, but it bears interesting differences with the suppressive effect of high levels of UV radiation, as shown by some recent studies. The aim of this article is to put some ideas in perspective and pose some questions within the field of photoimmunology based on established and new information, which may lead to new experimental approaches and, eventually, to a better understanding of the effects of sunlight on the human immune system.

Figure 1

Sunlight effects on the skin and the immune system. The effects triggered by different wavebands of radiation are summarized, focusing on those effects described in the text. UCA: urocanic acid; ROS: reactive oxygen species; MMPs: matrix metalloproteases; MED: minimal erythema dose; DCs: dendritic cells; LNs: lymph nodes.