Mechanism of Action of Vitamin A

Vitamin A (VA) is a fat-soluble agent that includes a cyclic ring, a polyene side chain, and a polar end group. The term VA typically includes retinol, retinaldehyde, retinoic acid (RA) isomers, and retinyl esters, such as retinyl palmitate, that are the primary forms of VA obtained from animal products. Mammals depend on diet to obtain enough VA once they are not able to synthesize this vitamin. VA plays an essential role in immunity, and its deficiency increases morbidity and mortality from some pathogens. Retinoids may be acquired as retinyl esters or ß-carotene and are absorbed in the small intestine, where they are carried to the liver by chylomicrons as retinyl-esters. Retinol increases the gut mucosal immunity, and all-trans-RA itself is vital to immune homeostasis due to the regulation of B-cell, T-cell, and dendritic cell action and balance in the release of anti-inflammatory cytokines. Retinoid levels in the skin are tightly controlled and are stored as retinyl esters.