Thymus

Function

The thymus is the grammar school of T-lymphocytes: a bilobed organ behind the sternum that, in childhood, seeds the body's entire T-cell repertoire. Immature thymocytes enter its cortex and face the gauntlet — only cells that recognise self-MHC but not self-antigens survive (positive and negative selection); over 95% die by design, preventing autoimmunity.

After puberty it involutes into fat, its work done — the T-cells it trained patrol for decades. Myasthenia gravis, the neuromuscular autoimmune disease, is associated with thymic abnormalities.

Blood supply

Internal thoracic and inferior thyroid branches.

Nerve supply

Vagus and sympathetic fibres modulate the microenvironment.

Clinical relevance

Myasthenia gravis

Thymic hyperplasia or thymoma accompanies most cases; thymectomy can induce remission.

Thymoma

The commonest anterior mediastinal tumour of adults — paraneoplastic syndromes accompany it.