The process of multipotential lymphatic stem cell maturation and differentiation into immunocompetent T cells is accomplished by the expression and deletion of specific surface CD antigens. The CFU-Lstem cells enter the medulla of the thymus via a post capillary venule and then migrate to the periphery of the thymic lobule. The presence of CD2 and CD7 molecules on the cell surface indicates an early stage of differentiation. This is followed by expression of the CD1 molecule, indicating the midstage of Tcell differentiation. As maturation progresses, the cells express TCRs, CD3, CD4, and CD8 molecules. It seems that intensity of interreacion between TCR/co receptor molecule complex and self peptide/MHC complex determine the outcome of the thymocite selection process. If the lymphocyte recognizes self MHC and self or foreign antigen, it will survive the selection (positive selection); if not, death of the cell will occur. Cells that pass the positive selection test leave the cortex and enter the medulla. Here they undergo another selection process in which cells directed to selfantigen displayed by self MHC are eliminated (negative selection). Cells that survive that selection then become either cytotoxic CD8+ Tlymphocytes or helper CD4+ Tlymphocytes.

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