Thymus

About the Thymus

The thymus is a soft, two lobed lymphoid organ in the upper front part of the chest. It is relatively large in infancy and childhood, continues to grow until puberty, and then gradually shrinks and is replaced by fat, a process called involution. Its function was established by Jacques Miller in the early 1960s. Immature T cells arrive from the bone marrow and pass through the outer cortex and inner medulla of the thymus, where they are selected: cells that cannot recognize the molecules of the body itself are eliminated, as are those that react too strongly to self antigens, a process essential for preventing autoimmunity. Only a small fraction survive to become mature T cells. Thymic epithelial cells express a wide range of proteins from other tissues under the control of the AIRE gene to support this selection. Absence of the thymus, as in complete DiGeorge syndrome, causes severe immunodeficiency. Abnormalities of the thymus, including thymomas, are associated with myasthenia gravis.