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

thymus An organ, present only in vertebrates, that is concerned with development of lymphoid tissue, particularly the white blood cells involved in cell-mediated immune responses (see T cell). 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.

thy·mus / ˈ[unvoicedth]īməs/ (also thy·mus gland) • n. (pl. -mus·es or -mi / -mī/ ) a lymphoid organ situated in the neck of vertebrates that produces T cells for the immune system. The human thymus becomes much smaller at the approach of puberty.

thymus gland One of the endocrine glands, located in the upper chest in mammals. In childhood, it controls the development of lymphoid tissue and the immune response to infection. Disorder of the thymus may be associated with autoimmune diseases (those caused by the body's own antibodies).See also endocrine system

thymus (th'y-mŭs) n. a bilobed organ in the root of the neck, above and in front of the heart. In relation to body size the thymus is largest at birth. It doubles in size by puberty, after which it gradually shrinks, its functional tissue being replaced by fatty tissue. In infancy the thymus controls the development of lymphoid tissue and immune response to microbes and foreign proteins. Its function in the adult is unclear.
—thymic adj.