Thymocyte

A thymocyte is an immature T cell developing in the thymus. It rearranges its antigen-receptor genes and undergoes selection before becoming a mature T cell or dying.

T-cell receptor gene rearrangement: Somatic DNA recombination that assembles functional T-cell receptor genes in developing T cells. Thymocytes build their antigen receptors through this process before selection tests them.
Thymus: A primary lymphoid organ where T cells develop and acquire central immune tolerance. It houses the stromal cells and compartments that guide thymocyte development.
Central tolerance: Immune self-tolerance established by eliminating or altering self-reactive lymphocytes during development in primary lymphoid organs. Thymocyte selection is a major mechanism for establishing central tolerance in T cells.
DiGeorge syndrome: A congenital disorder, usually involving 22q11.2 deletion, that can cause thymic hypoplasia and T-cell deficiency. An underdeveloped thymus can leave too few thymocytes to generate a normal T-cell population.
Double-negative thymocyte: An early thymocyte lacking both CD4 and CD8 coreceptors. This stage precedes productive T-cell receptor beta-chain rearrangement and the double-positive stage.
T-cell receptor: A receptor on T cells that recognizes peptide antigens presented by major histocompatibility complex molecules. Thymocyte selection depends on the specificity and binding strength of this receptor.
Regulatory T cell: A T-cell subset that suppresses immune responses and helps maintain self-tolerance. Some self-reactive thymocytes develop into regulatory T cells instead of being deleted.
Severe combined immunodeficiency: A group of inherited disorders causing profound defects in T-cell development and often other lymphocyte functions. Several forms interrupt signals required for thymocyte survival and maturation.
Double-positive thymocyte: A thymocyte expressing both CD4 and CD8 coreceptors. Most thymocytes reach this stage before positive and negative selection.
Major histocompatibility complex: A family of cell-surface proteins that presents peptide fragments for recognition by T cells. Thymocytes must recognize self MHC to survive positive selection.

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