Sequential recruitment of antibody class‐committed B lymphocytes during ontogeny

Chickens were bursectomized surgically during the first week after hatching and immunized subsequently with human serum albumin (HSA). Bursectomy (Bx) 4 days posthatching suppressed the IgG response, but did not affect the IgM response when tested at 2 weeks. The overall responsiveness had declined by 4 weeks, however, probably due to the nonreplicating character of the early peripherally seeded B cells. Furthermore, these chickens failed to give a memory response. B cells with self‐replicating and memory potential are seeded later than 4 days posthatching. However, 7 day Bx still produced a selective suppression of the primary IgG response while the IgM response, although delayed, reached a normal peak titer. Treatment of memory cells in vitro with anti‐μ serum interfered with the adoptive transfer of both IgM and IgG responses. The data are interpreted in terms of sequential seeding of IgM and IgG precursors of the anti‐HSA clones, both of which maintain membrane‐bound μ‐chain antigen.