Counting components of the chicken's B cell system.

At the risk of representing a chicken as a hybrid between a hummingbird and an ostrich, we can summarize the preceding sections and order-of-magnitude estimates as follows. Chicken B lymphocytes are derived from less than 10(5) lymphoid precursor cells, which either have already rearranged their Ig genes before they colonize the embryonic bursa, or (more probably) rapidly give rise to cells with rearranged genes within the bursa's 10(4) follicles. Since the bird's functional germline Ig V genes are few in number (less than or equal to 10?), most rearrangements have similar outcomes. The B cells proliferate rapidly in the bursa, in an antigen-independent manner, undergoing somatic modifications of their Ig V genes at a high rate (probably at least once in every 10(3) cell divisions). In the young chick, B cells are produced in the bursa at a rate of 10(7) to 10(8) per d; many of these die but the rest contribute to formation of the adult bird's B cell pool of about 10(10) lymphocytes, with a repertoire of at least 10(6) different antibody specificities. the bird's B cells are entirely self-renewing, in the sense that none are derived from Ig-negative precursors at any time after hatching.