This lies in the upper part of the chest. The centre (cortex) resembles LYMPHOID TISSUE and is made up of masses of small round cells called thymocytes (see THYMOCYTE; LYMPHOCYTE). The medulla is more loosely cellular and consists of a stroma which contains far fewer lymphocytes than in the cortex.

The thymus gland is a vital part of the immunological system. Stem cells (see STEM CELL) from the BONE MARROW come to the thymus where they develop into immunologically competent cells. There are two distinct populations of lymphocytes. One is dependent on the presence of the thymus (T-lymphocytes); the other is independent of the thymus (B-lymphocytes). Both are concerned with immune responses (see IMMUNITY). The T-lymphocyte is a cell which in the absence of antigenic stimulation (see ANTIGEN) circulates through the blood, lymph nodes and back into the circulation again over a period of more than ten years. It performs a policing role, awaiting recognition of foreign material which it is able to identify as such. It reacts by multiplication and transformation and these are the ingredients of the immune response. B-lymphocytes are produced in the bone marrow and are concerned with the production of circulating ANTIBODIES.

The most common clinical disorder associated with abnormality of the thymus is MYASTHENIA GRAVIS. Ten per cent of patients with myasthenia gravis will have a tumour of the thymus, whilst the remainder will have inflammatory changes in the thymus called thymitis.