Malnutrition, urocanic acid, and sun may interact to suppress immunity in sojourners to high altitude

Irradiation of skin by ultraviolet radiation in mice and humans leads to a suppression of cell-mediated immunity. This process is initiated when one o f the photoreceptors in skin, trans-urocanic acid, is photoisomerized to ci s-urocanic acid, an immunomodulator. High levels of L-histidine, histamine, and trans-urocanic acid are found in humans and animals when they are prot ein malnourished. Mice fed on an elevated L-histidine diet have more trans- urocanic acid in the skin and are more susceptible to UV-induced immune sup pression. Sojourners to high altitudes are malnourished, suffer protein cat abolism, are exposed to sun, and often acquire infectious diseases. There i s evidence that sunscreens may not adequately protect the immune system. Fu rthermore, UV intensity increases with altitude. We propose a testable hypo thesis: UV radiation causes photoimmune suppression in sojourners to high a ltitude and this allows infectious diseases to develop. The mechanism we pr opose includes protein malnutrition, high levels of trans-urocanic acid, ul traviolet radiation, formation of cis-urocanic acid, immune suppression, an d infection.