Development of T Cells

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The page below is a sample from the LabCE course An Update on Basic Concepts of Immunity. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Development of T Cells

Many similarities exist between the development of B Cells and T Cells. The most obvious is that the exact same mechanism of gene recombination facilitated by RAG occurs, which creates great diversity among T Cell receptors, just as it does among B Cell receptors.
However, there are some notable differences. Although T cells derive from the Lymphoid Progenitor, as do B cells, they migrate to the thymus to develop and mature. Gene recombination occurs in T Cells while they are in the thymus. Also, as they begin to mature they start out as "double negative" cells (developing T cells are often referred to as "thymocytes") which means they have neither CD 4 or CD 8 on their surface. As development continues they will recognize either an MHC I or MHC II molecule on other cells in the thymus. Depending on which one is recognized, that receptor will survive and the other one will disappear. For instance, if MHC II is recognized by the CD4, it will lose the CD 8 and so be destined to become a T Helper cell. This process is referred to as "positive selection".
As with B cells, however, it would be detrimental to recognize and respond too strongly to self-antigens. So once the cell becomes a single positive (either CD 4 or CD 8) and it then attaches to an MHC presenting self-antigen, that process will trigger death by apoptosis. Once again, this process is called negative selection. If it survives this process, it can exit the thymus and travel to the lymph nodes where it can become activated.