thymus

thymus The thymus has had a varied history in terms of attribution of function, from at least the time of Galen, who named it the seat of the soul.

It is a relatively large organ at birth, centrally placed in the upper chest behind the sternum, and extending into the neck. It grows with the child until puberty, then regresses to almost nothing in adulthood. It consists mostly of lymphocytes. Up until the early twentieth century, an excessively large thymus was from time to time held responsible for some unexplained infant deaths, as part of so-called status lymphaticus believed to be a generalized superfluity of lymphatic tissue. This theory was discredited in the 1930s. Since then, the role of the thymus as part of the lymphatic system has become better understood, along with the study of the mechanisms of immunity, of autoimmune disease, and of factors causing rejection of transplants.

The thymus is now known to be involved in processing lymphocytes, which are crucial for the ‘cellular component’ of the immune response and also assist the provision of the ‘humoral component’ — the antibodies in the blood. It is a highly active organ in the young body, and the unique site for the selection of lymphocytes that will be ‘competent’ for this role in the immune response and for the maturation that prepares them for it.

Some of the lymphocytes that originate in the bone marrow move early in life to the thymus. These are the candidates for giving rise to T-lymphocytes, if their progeny emerge as the few survivors of a rigorous selection process, according to the appropriateness of the cell membrane receptors that result from random rearrangement of their genes. This process takes place in the outer layer, the ‘cortex’, of the young thymus, where lymphocytes are densely packed and actively dividing. Those that develop receptors that are able to bind the right sort of peptides proliferate; the others die. The ‘good’ ones proceed to the central ‘medulla’ and are further weeded out, until only those that have ‘learnt’ to bind, ‘non-self’ peptides remain; rejects at this stage are those which could bind the body's own ‘self’ peptides, and would therefore be liable to cause autoimmune diseases.

The T-lymphocytes that leave the thymus enter the circulation, and some settle in lymphoid tissue, including the lymph nodes and the spleen. Their further story is that of the immune response: subsets of T-cells take part in several different ways, including the activation of phagocytic cells, killing specific antigen-producing cells, and regulating the production of antibodies by the B-lymphocytes (those which have not been processed by the thymus).

The thymus has sometimes been surgically removed in the treatment of myasthenia gravis, in which antibodies are formed to the body's own acetylcholine receptors in skeletal muscle, but in most instances there are other, more appropriate treatments.

Sheila Jennett

See also autoimmune disease; immune system; lymphatic system.