The thymus is made up of immature T cells called thymocytes, as well as lining cells called epithelial cells which help the thymocytes develop. T cells that successfully develop react appropriately with MHC immune receptors of the body (called positive selection) and not against proteins of the body (called negative selection). The thymus is the largest and most active during the neonatal and pre-adolescent periods. By the early teens, the thymus begins to decrease in size and activity and the tissue of the thymus is gradually replaced by fatty tissue. Nevertheless, some T cell development continues throughout adult life.
the thymus is made up of immature t cells called thymocytes as well as lining cells called epithelial cells which help the thymocytes develop t cells that successfully develop react appropriately with mhc immune receptors of the body (called positive selection) and not against proteins of the body (called negative selection) the thymus is the largest and most active during the neonatal and pre-adolescent periods by the early teens the thymus begins to decrease in size and activity and the tissue of the thymus is gradually replaced by fatty tissue nevertheless some t cell development continues throughout adult life