"Vitamin A helps cells reproduce normally- a process called differentiation. Vitamin A, by maintaining healthy cell membranes, helps prevent infections. Beta-carotene, a substance from plants that the body can convert into Vitamin A, also acts as an antioxidant.
Retinoic acid is needed for the differentiation of basal cells into mucus epithelialcells. A deficiency results in keratinization of mucus membranes. This decreases the protective barrier role played by these membranes resulting in an increased number of infections and other pathologies. Vitamin A maintains the integrity of the mucus membranes and, thus, is vital for protecting the body against bacterial, viral or parasitic infections."