Antigen-specific immune response impairment in the chick as influenced by dietary vitamin A

The effects of a broad range of dietary vitamin A levels on antigen-specific immune responses of the chick were studied. Vitamin A deficiency and long-term excess caused impairment of the immune response as demonstrated by antigen-specific antibody production and T-lymphocyte proliferation in vitro. Immune responsiveness was correlated with both hepatic and blood vitamin A levels, and was affected prior to other physiological manifestations of hypo- or hypervitaminosis. In contrast to the effects of long-term vitamin excess, a large vitamin A bolus did not cause immune response impairment and, on the contrary, restored normal immune functions in previously vitamin A-depleted chicks. This finding implied a regulatory role for vitamin A in the immune system rather than a constitutive one.