An organ, present only in vertebrates, that is concerned with development of lymphoid tissue, particularly the white blood cells involved in cell-mediated immunity (see T cell; thymocyte). In mammals it is a bilobed organ in the region of the lower neck, above and in front of the heart. The thymus undergoes progressive shrinkage (involution) throughout life, starting after the first 12 months. Haemopoietic stem cells from the bone marrow migrate to the thymus, attracted by chemotactic factors, and begin to divide and differentiate to form the many subpopulations of T cells. As their progeny cells migrate through the thymus from its cortex to medulla, they interact with thymic ‘nurse cells’ and with each other and are influenced by various extracellular proteins and thymic peptide hormones (e.g. thymosin and thymopoietin). All these factors help to promote the differential expression of surface antigens and development of distinctive immunological competences.