Vitamin A plays a vital regulating role in the immune system. Vitamin A deficiency leads to a loss of ciliated cells in the lung, an important first line defense against pathogens. Vitamin A promotes mucin secretion and microvilli formation by mucosa, including the gastrointestinal tract mucosa. Vitamin A regulates T-cell production and apoptosis (programmed cell death) (Nutrition Reviews 1998;56:S38-S48).