Thymus

The thymus is a lymphoid organ behind the breastbone where immature T cells mature and undergo selection. It helps establish immune tolerance by eliminating many T cells that react strongly to the body’s own molecules.

T-cell development: The process by which T-cell precursors acquire the receptors and functions needed for adaptive immunity. Thymic maturation is the central tissue phase of this broader developmental process.
T cell: A lymphocyte that recognizes antigen through a T-cell receptor and coordinates or carries out immune responses. T cells are the mature products of thymic development.
DiGeorge syndrome: A congenital disorder, often caused by a 22q11.2 deletion, that can include thymic hypoplasia and T-cell deficiency. Underdevelopment or absence of the thymus can impair T-cell immunity.
Bone marrow: The tissue within bones that produces blood cells and supports B-cell development in humans. Bone marrow generates T-cell precursors, while the thymus is where they mature.
Third pharyngeal pouch: An embryonic structure that contributes to the thymus and inferior parathyroid glands. The thymus develops from the third pharyngeal pouch during embryogenesis.
Positive selection: A developmental checkpoint that preserves T cells able to recognize the body's major histocompatibility molecules. Thymic cortical cells test whether developing T cells can engage self-MHC.
Thymocyte: An immature T-lineage cell developing within the thymus. Thymocytes pass through the organ's maturation and selection stages.
Thymoma: A tumor arising from thymic epithelial cells, sometimes associated with autoimmune disorders. Thymic tumors can disrupt immune regulation and are linked to myasthenia gravis.
Lymph node: A small lymphoid organ that filters lymph and supports immune responses to antigens. Lymph nodes activate mature lymphocytes rather than carrying out thymic selection.
Jacques Miller: An Australian immunologist whose experiments established the thymus's role in immune development. Miller showed that removing the thymus from young mice causes profound immune defects.

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