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T cells are central components of adaptive immunity and are responsible for cell-mediated immune responses such as killing infected cells and regulating immune reactions. T-cell development begins in the bone marrow from hematopoietic stem cells and is completed in the thymus, where precursor cells differentiate into immunocompetent but self-tolerant mature T lymphocytes. Progenitor cells migrate from bone marrow to the thymus and are called thymocytes, which then undergo stepwise maturation controlled by gene rearrangement, surface marker expression, and selection processes. ([immunology.org][1]) The earliest thymocytes are known as double negative (DN) cells because they do not express CD4 or CD8 co-receptors. These cells progress through DN1 to DN4 stages, characterized by markers such as CD44 and CD25. During the DN3 stage, beta selection occurs, where successful rearrangement of the T-cell receptor (TCR) _____ ______ ________ ________ ______.
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