the B cell lineage, which produces copious antibody, but cannot divide and eventually dies after a few days or weeks. The progenitor CLP of the mouse or the progenitor MLP of the human differentiates into lymphocytes by first becoming a lymphoblast (Medical Immunology, p.10). It then divides several more times to become a prolymphocyte that has specific cell-surface markers unique to either a (1) T cell or (2) B cell. The progenitor can also differentiate into (3) natural killer cells (NKs) and (4) dendritic cells (DCs). T cells, B cells and NK cells (and all other Innate lymphoid cells) are unique to the lymphocyte family, but dendritic cells are not. DCs that are of identical appearance but have different markers are spread throughout the body, and come from either lymphoid and myeloid lineages, but these cells may have somewhat different tasks and may take up lodging preferentially in different locations. (Revise in light of new research) This is now an open question; also, the different dendritic cell lineages may have different ‘tasks’ or functions, and stay in different ‘locations.’ T and B lymphocytes are indistinguishable under the microscope. The inactive B and T cells are so featureless with few cytoplasmic organelles and mostly inactive chromatin that until the 1960s textbooks could describe these cells, now the central focus of immunology, as having no known function! However T and B lymphocytes are very distinct cell lineages and they ‘grow up’ in different places in the body. They perform quite different (although co-operative) functions in the body. No evidence has ever been found that T and B cells can ever interconvert. T and B cells are biochemically distinct and this is reflected in the differing markers and receptors they possess on their cell surfaces. This seems to be true in all vertebrates, although there are many differences in the details between the species. Regardless of whether the CLP (mouse) or MLP or a small closely related set of progenitor cells take credit for generating the profusion of lymphocytes, the same lymphoid progenitors can still generate some cells that are clearly identifiably myeloid. Lymphopoiesis for T cells T cells are formed in bone marrow then migrate to the cortex of the thymus to undergo maturation in an antigen-free environment for about one week where a mere 2–4% of the T cells succeed. The remaining 96–98% of T cells die by apoptosis and are