Thymus: The Organ Where T Cells Mature

The thymus is a primary lymphoid organ located in the anterior superior mediastinum. It is essential for T lymphocyte (T cell) development and selection.

Key Functions

• T cell maturation: Bone marrow-derived progenitors enter the thymus cortex → proliferate, rearrange TCR genes → undergo positive selection (recognize self-MHC) and negative selection (eliminate strong self-reactivity).
• Naïve T cell export: Mature CD4⁺ and CD8⁺ T cells exit to populate peripheral lymphoid organs.
• Endocrine activity: Secretes thymosin, thymopoietin, thymulin—hormones promoting T cell differentiation and function.
• Central tolerance: Deletes autoreactive T cells; generates regulatory T cells (Tregs).

Lifecycle

• Largest and most active in infancy/childhood.
• Progressive involution (fatty replacement) begins at puberty; continues throughout life.
• Residual function persists; thymic output declines but never fully ceases.

Clinical Relevance

• DiGeorge syndrome (22q11.2 deletion): Thymic aplasia/hypoplasia → severe T cell deficiency.
• Thymoma: Thymic epithelial tumor; associated with myasthenia gravis, pure red cell aplasia, hypogammaglobulinemia (Good's syndrome).
• Stress/glucocorticoids: Cause transient thymic atrophy.

Without the thymus, adaptive immunity (cell-mediated) fails. It is the "school" where T cells learn self vs. non-self.