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Definition of THYMUS
: a glandular structure of largely lymphoid tissue that functions especially in the development of the body's immune system, is present in the young of most vertebrates typically in the upper anterior chest or at the base of the neck, and tends to atrophy in the adult
: a glandular structure of largely lymphoid tissue that functions in cell-mediated immunity by being the site where T cells develop, that is present in the young of most vertebrates typically in the upper anterior chest or at the base of the neck, that arises from the epithelium of one or more embryonic branchial clefts, and that tends to disappear or become rudimentary in the adult—called also thymus gland
Pyramid-shaped lymphoid organ (seelymphoid tissue) between the breastbone and the heart. Starting at puberty, it shrinks slowly. It has no lymphatic vessels draining into it and does not filter lymph; instead, stem cells in its outer cortex develop into different kinds of T cells (seelymphocytes). Some migrate to the inner medulla and enter the bloodstream; those that do not may be destroyed to prevent autoimmune reactions. This process is most active during infancy. If a newborn's thymus is removed, not enough T cells are produced, the spleen and lymph nodes have little tissue, and the immune system fails, causing a gradual, fatal wasting disease. Thymus removal in adults has little effect.