Carotenoids and the immune response.

There is growing evidence from in vitro and in vivo laboratory animal studies that beta-carotene can protect phagocytic cells from autooxidative damage, enhance T and B lymphocyte proliferative responses, stimulate effector T cell functions, and enhance macrophage, cytotoxic T cell and natural killer cell tumoricidal capacities, as well as increase the production of certain interleukins. Many of these effects have also been seen with carotenoids lacking provitamin A activity but having the antioxidant and singlet oxygen quenching capacities of beta-carotene. The association of immunoenhancement with decreased tumor burden in animals given carotenoids suggests a potential explanation for the epidemiological data linking lower carotenoid status with higher incidences of certain cancers. Since vitamin A is a relatively poor antioxidant and cannot quench singlet oxygen, beta-carotene may have more importance as a nutrient than simply serving as a precursor of vitamin A.