in blood tests) they are certainly not inert but can and do get around the body to anywhere there is a need; and when such need arises, new rounds of 'downstream' lymphopoiesis such as cell multiplication and differentiation may arise, coupled with intense mitotic and metabolic activity. This is hardly a simple topic. In his 1976 text Immunology, Aging and Cancer immunologist and Nobel Prize winner Sir Frank Macfarlane Burnet speculated that the immune system might one day be found to be as complex as the nervous system. As the production of lymphocytes is so close to the central role of the immune response it is wise to approach the study of it with some humility in the face of the task, although there are general principles that help in understanding. Process Lymphopoiesis can be viewed in a mathematical sense as a recursive process of cell division and also as a process of differentiation, measured by changes to the properties of cells. Given that lymphocytes arise from specific types of limited stem cells - which we can call P (for Progenitor) cells - such cells can divide in several ways. These are general principles of limited stem cells. Considering the P as the ‘mother’ cell, but not a true stem cell, it may divide into two new cells, which are themselves identical, but differ to some degree from the mother. Or the mother cell P may divide unequally into two new daughter cells both of which differ from each other and also from the mother. Any daughter cell will usually have new specialized abilities and if it is able to divide it will form a new sub-lineage. The difference of a daughter cell from the mother may be great, but it could also be much less, even subtle. What the P mother cell does not do is divide into two new P mother cells or a mother and a daughter; this is a matter of observation as such limited progenitor cells are known to not self-renew. There is a sort of exception when daughter cells at some level of the lineage may divide several times to form more seemingly identical cells, but then further differentiation and division will inevitably occur, until a final stage is reached in which no further division can occur and the cell type lineage is finally mature. An example of maturity is a plasma cell, from