Thymic

The term thymic is an adjective relating to the thymus, a lymphoid organ located in the anterior mediastinum. In humans and many mammals, the thymus plays a central role in the immune system by supporting the development and maturation of T lymphocytes (T cells). The organ is organized into cortex and medulla, populated by developing thymocytes and thymic epithelial cells. During T cell maturation, cells undergo positive selection in the cortex to recognize self-MHC molecules and then negative selection in the medulla to reduce autoreactivity, resulting in a repertoire of self-tolerant T cells that exit to peripheral lymphoid tissues. Traditionally described thymic hormones, including thymosin and thymopoietin, were reported historically; modern understanding emphasizes signaling from thymic epithelial cells and cytokines such as interleukin-7 as drivers of T cell development.

Thymic activity is high in infancy and early childhood, supporting rapid T cell production. Beginning around adolescence, the thymus undergoes involution, gradually shrinking and replacing lymphoid tissue with adipose tissue; this reduces new T cell output but peripheral T cells maintain immune function through memory pools.

Clinical relevance: Thymic neoplasms include thymoma and thymic carcinoma. Thymic hyperplasia can occur in autoimmune conditions such as myasthenia gravis and pure red cell aplasia, and thymic tumors can be associated with Good's syndrome (thymoma with hypogammaglobulinemia). Thymic cysts and ectopic thymic tissue are additional non-neoplastic conditions. Treatment may involve thymectomy in selected cases, especially for myasthenia gravis.