Vitamin A, a generic designation for an array of organic molecules that includes
retinal,
retinol and
retinoic acid, is an essential nutrient needed in a wide array of aspects including the proper functioning of the visual system, maintenance of cell function and differentiation, epithelial surface integrity, erythrocyte production, reproduction, and normal immune function.
Vitamin A deficiency is one of the most common
micronutrient deficiencies worldwide and is associated with defects in adaptive immunity. Reports from epidemiological studies, clinical trials and experimental studies have clearly demonstrated that
vitamin A plays a central role in immunity and that its deficiency is the cause of broad immune alterations including decreased humoral and cellular responses, inadequate immune regulation, weak response to
vaccines and poor lymphoid organ development. In this review, we will examine the role of
vitamin A in immunity and focus on several aspects of T cell biology such as T helper cell differentiation, function and homing, as well as lymphoid organ development. Further, we will provide an overview of the effects of
vitamin A deficiency in the adaptive immune responses and how
retinoic acid, through its effect on T cells can fine-tune the balance between tolerance and immunity.