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, B cells 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 CD4 nor CD8 on their surface. As development continues, they will recognize either an MHC I or MHC II molecule in other cells in the thymus. Depending on which one is identified, that receptor will survive, and the other will disappear. For instance, if MHC II is recognized by CD4, it will lose CD8 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 CD4 or CD8) and 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.
