The thymus is a soft, pinkish-gray organ positioned in the superior mediastinum, right behind the sternum and directly in front of the heart. It sits safely in the chest cavity, uniquely large during infancy and childhood relative to body size. Structurally, a delicate connective tissue capsule wraps around the thymus, sending inward extensions called trabeculae that divide the organ into distinct lobules. Each lobule features two primary zones: an outer, densely packed cortex and an inner, lighter-staining medulla. This specialized layout creates a controlled microenvironment where developing immune cells can mature away from general blood circulation.
Immature lymphocytes, known as thymocytes, travel from the bone marrow to the thymus to complete their development into functional T-lymphocytes. Inside the thymic cortex and medulla, these cells undergo rigorous training through two major selection steps:
Only a small percentage of original thymocytes survive this strict screening process to enter the bloodstream as mature, naive T-cells.
Beyond acting as a cellular training ground, the thymus functions as an endocrine gland. Epithelial reticular cells within the organ secrete vital hormones such as thymosin, thymopoietin, and thymulin. These chemical messengers promote the maturation of T-cells locally within the organ and help support overall immune system coordination throughout the body. While the thymus decreases in size with age, these hormonal outputs help maintain baseline immune defense capacity as a person grows older.
The thymus reaches its maximum size relative to body weight during late childhood or puberty. Following puberty, the organ undergoes a natural, gradual shrinking process known as age-related thymic involution. Active lymphoid tissue slowly gets replaced by adipose and connective tissue over decades. Although the output of new naive T-cells drops significantly in older adults, the body maintains its established memory T-cell pool to fight off common infections.
Pathology involving the thymus can significantly impact overall health. Thymomas are tumors originating from the epithelial cells of the thymus, which are frequently linked to the autoimmune muscle-weakening disorder myasthenia gravis. In this condition, abnormal immune responses involve antibodies attacking acetylcholine receptors at the neuromuscular junction. Additionally, rare genetic disorders like DiGeorge syndrome involve congenital absence or underdevelopment of the thymus, leading to severe primary immunodeficiency.