Many similarities exist between the development of B Cells and T Cells. The most obvious is that the same mechanism of gene recombination facilitated by RAG occurs, creating 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 as "double negative" cells (developing T cells are often referred to as "thymocytes") which means they have neither CD 4 nor 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 CD 4, it will lose CD 8 and be destined to become a T Helper cell. This process is referred to as "positive selection."
However, as with B cells, 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.
