Vitamin A plays a role in the immune system by protecting cells from free radicals, but its most critical function is supporting eyesight.
How it works in the body
Rod cells, a type of photoreceptor cell in the eye, utilize a protein called rhodopsin to detect light. Rhodopsin is itself composed of the protein opsin coupled with a derivative of vitamin A known as an 11-cis-retinal. When rhodopsin absorbs light, the 11-cis-retinal molecule is transformed to a different type of retinoid, known as all-trans-retinal. The transformation triggers the signaling of visual neuron cells, which transmit the corresponding visual stimulus to the brain.
Vitamin A plays roles in a variety of other processes in the body, including immune function, apoptosis (apoptosis is a programmed removal of unneeded cells) and cell-to-cell communication. Vitamin A's primary connection to these various processes lies in its ability to regulate which genetic instructions (which parts of a cell's DNA) are acted upon by the cell.,
Vitamin A compounds are absorbed in the small intestine and transported, like other fat-soluble vitamins, to the liver where 80 percent of vitamin A is found.
 Carotenoids, a class of pigments commonly found in carrots, are a type of provitamin A compound.
 Rhodopsin is a member of the G-Protein Receptor Family.
 11-cis-retinal is a cofactor (a non-protein compound that is critical to a protein's function) of rhodopsin.
 "Vitamin A." In Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc, 85. Washington, D.C.: National Academies Press, 2001. http://plk.tn/1HMaEkp
 This is referred to as gene expression.
 Stipanuk, Martha H., and Marie A. Caudill. "Vitamin A." In Biochemical, Physiological, and Molecular Aspects of Human Nutrition, 686. Third Edition ed. St. Louis: Elsevier, 2013.
 Erdman, John W., Ian MacDonald, and Steven H. Zeisel. "Vitamin A." In Present Knowledge in Nutrition, 154. Washington, DC: International Life Sciences Institute, 2012.