Vitamin A and its metabolites are required for normal functioning of the immune system. The skin and mucosal cells (cells that line the airways, digestive tract, and urinary tract) function as a barrier and form the body's first line of defense against infection. Vitamin A maintains the integrity and function of these cells; therefore, deficiency in this micronutrient results in loss of integrity of the mucosal barriers and increased vulnerability to respiratory and diarrheal diseases and other infections. The vitamin A derivative, retinoic acid, functions as a hormone to regulate more than 500 genes, several of which are involved in controlling the production, proliferation, and differentiation of immune cells, including neutrophils, monocytes, and lymphocytes. Thus, vitamin A deficiency compromises the function of cell-mediated immunity and also impairs antibody responses to antigens, adversely affecting humoral immunity. Additionally, vitamin A deficiency adversely affects components of innate immunity, such as phagocyte function and the activity of so-called natural killer cells, and alters cytokine signals that influence various immune responses. Vitamin A deficiency is common worldwide, especially in developing nations, and has detrimental effects on maternal and child health. Severe vitamin A deficiency in children increases morbidity and mortality related to measles, malaria, and diarrheal infections; mild vitamin A deficiency may increase rates of certain infections, as well. Vitamin A supplementation can enhance immunity, decrease susceptibility to infection, and significantly reduce infection-related morbidity and mortality in children.