 T cells are formed in bone marrow then migrate to the cortex of the thymus to undergo maturation in an antigen-free environment for about one week where a mere 2-4% of the T cells succeed.The remaining 96-98% of T cells die by apoptosis and are phagocytosed by macrophages in the thymus.So many thymocytes (T cells) die during the maturation process because there is intensive screening to make sure each thymocyte has the ability to recognize self-peptide