represent the dominant T cell population. These ‘non-MHC restricted T cells’ are involved in specific primary immune responses, tumor surveillance, immune regulation and wound healing. Several differences between αβ and γδ T cell development have been described. They emigrate from the thymus in "waves" of clonal populations, which home to discrete tissues. For example, one kind is found in the peripheral blood while another predominates in the intestinal tract. Natural Killer T cells Human NKT cells are a unique population and are thought to play an important role in tumor immunity and immunoregulation. T Regulatory cells Treg cells are considered as naturally occurring regulatory T cells. Tregs comprised about 5% of the circulating CD4+ T cells. These cells are thought to possess an important autoimmunity property by regulating 'autoreactive' T cells in the periphery. (Medical Immunology, p. 117-122) Lymphopoiesis for B cells B cells are formed and mature in bone marrow (and spleen). It is a good mnemonic aide that B cells are formed in the bone marrow, but it is a mere coincidence since B cells were first studied in the chicken's bursa of Fabricius and it is from this bursa that B cells get their name. These B cells then leave the bone marrow and migrate via bloodstream and the lymph to peripheral lymphoid tissues, such as a spleen, lymph nodes, tonsils and mucosal tissues. Once in a secondary lymphoid organ the B cell can be introduced to an antigen that it is able to recognize. Through this antigen recognition and other cell interactions the B cell becomes activated and then divides and differentiates to become a plasma cell. The plasma cell, a B cell end product, is a very active antibody-secreting cell that helps protect the body by attacking and binding to antigen. Even after many decades of research, some controversy remains as to where B cells mature and 'complete their education', with the possibility remaining that the site may also partially be peri-intestinal lymphoid tissues. B lymphopoiesis occurs exclusively in the bone marrow and B lymphocytes are made continuously throughout life there in a 'microenvironment' composed of stromal cells, extracellular matrix, cytokines and growth factors, which are critical for proliferation, differentiation, and survival of early lymphocyte and B-lineage precursors. The relative proportion of precursor B cells in the bone marrow remains rather constant throughout the life span of an organism. There are stages such as Pre-B-I