The Evolution of B-Cell Clones

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Abstract
Large numbers of B cells are produced throughout life as the result of both primary B lymphopoiesis (Opstelten and Osmond 1983) and antigen-driven B- cell proliferation (Fliedner et al. 1964). It is a feature of the system that a high proportion of the cells produced have a brief lifespan (Kumararatne et al. 1985; Freitas et al. 1986; MacLennan and Gray 1986). Some, however, survive for much longer following positive selection. This can be antigen-dependent (MacLennan and Gray 1986; Gray et al. 1986). but may also be antigen- independent (MacLennan 1987). The B cells which migrate between the follicles of secondary lymphoid tissues and those which are located in the marginal zones of the spleen have been positively selected in this way. These cells, which are not in cell cycle, have an average lifespan of around 1 month (Gray 1988a; Gray and Skarvall 1988). If they are not activated by antigen within this period they die.