CD4 and vitamin D deficiency: This effect of light is likely to be mediated at least partly by vitamin D for the following reasons: (1) within the immune system, the cells expressing the greatest concentration of the vitamin D receptor are activated CD8+ T cells75; (2) vitamin D augments the mitogen‐induced proliferation of CD8+ T cells and lowers the CD4/CD8 ratio in vitro76; (3) administration of vitamin D increases the CD8+ T‐cell count77; and (4) in vitamin D deficiency, the proportion of CD8+ T cells in the blood decreases and the CD4/CD8 ratio increases.78