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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.
This entry comes from Encyclopædia Britannica Concise. For the full entry on thymus, visit Britannica.com.