cells (5% to 10% of the total); Pre-B-II cells (60% to 70%) while the remaining 20% to 25% are immature B cells. Most textbooks say that B Cells mature in the bone marrow but, generally, immature B cells migrate to the spleen for 'higher education' of some sort where they go through transitional stages before final maturation.(Medical Immunology, p. 136) B lymphocytes are identified by the presence of soluble immunoglobulin G (IgG). This is the most common protective immunoglobulin in the adult body. After antigenic stimulation, B cells differentiate into plasma cells that secrete large quantities of soluble IgG. This is the final stage of B lymphopoiesis but it is the clincher because the plasma cells must either issue antibody close to a source of infection, or disseminate it in the blood to fight an infection at a distance or in an inaccessible part of the body. Basic map of B cell lymphopoiesis A generally regarded valid map of B cell lymphopoiesis is as follows in sequence, in two parts with the first being in the bone marrow and the second in the spleen:. The development process in the bone marrow occurs in germinal centers In the bone marrow Pro-B Pre-B-I Pre-B-II large Pre-B-II small Imm(ature) In the spleen T1 T2/T3 (Marginal Zone (MZ); B-1; B-2) B-2 further differentiate into: (Germinal Center (GC); Memory; Plasma ) Lymphopoiesis for NK cells NK cells, which lack antigen specific receptors, develop in the bone marrow. After maturation and release from the marrow they circulate in the blood through their lifetime seeking opportunity. The opportunity they seek is to encounter and recognize and then kill abnormal cells such as cancer or virally infected cells. It is well known that lymphocytes never have granules or at least not granules that are readily visible even upon staining. However NK cells are the exception. They do have numerous granules which provide their ability to kill cells and these granules are why NK cells have an alternate name- Large Granular Lymphocytes (LGLs). NK cells are the only lymphocytes considered part of the innate immune system (in contrast to the adaptive immune system. Yet they are much more closely related to T cells (part of the adaptive immune system) than to other cells of the innate immune system. NK cells not only share many surface markers, functions and activities in common with T Cells, they also arise from a