The physical and functional heterogeneity of circulating lymphocyte populations

There is a considerable heterogeneity amongst populations of both fixed and circulating lymphocytes other than that determined by the organs from which these cells originate. The functional differences with which these cells are endowed during their primary differentiation are further modified by the changing environments they encounter during their life and by reassortitive processes occurring during their migration. The range of inductive and modulating stimuli applied to the free-floating and fixed lymphoid cell populations in different parts of the lymphatic apparatus is changing continually and this leads to patterns of cell differentiation which persist for variable periods of time. The experience of a variety of environmental interactions gives the lymphocyte a range of reactive options which are denied to those cells held in fixed relationships with other cells. As a consquence there is no predictable equivalence in the morphological or functional attributes of different lymphocyte populations present in different parts of the lymphatic system.