All-trans retinol is a diterpenoid component of vitamin A. This compound is an essential vitamin and is necessary for effective vision, skin health, and bone growth. Retinol and its metabolites activate retinoic acid receptors (RARs) and retinoid X receptors (RXRs), both of which have multiple isotypes and isoforms. During fetal development, retinol influences cellular differentiation in a variety of ways. Retinol also plays a role in cancer stem cell differentiation, mediated through ERK1/2 signaling. In the immune system, retinol appears to play a protective role in autoimmune diseases such as multiple sclerosis and encephalopathy, downregulating pro-inflammatory responses stimulated by Th1 and Th17 cells.