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In the first week of the primary immune response to the (4-hydroxy-3-nitrophenyl) acetyl (NP) hapten, plasmacytic foci and germinal centers (GCs) in C57BL/6 mice are comprised of polyclonal populations of B lymphocytes bearing the λ1 L-chain (λ1+). The Ig H-chains of these early populations of B cells are encoded by a variety of V H and D exons undiversified by hypermutation while later, oligoclonal populations are dominated by mutated rearrangements of the V H 186.2 and DFL16. 1 gene segments. To assess directly Ab affinities within these defined splenic microenvironments, representative VDJ rearrangements were recovered from B cells participating in the early immune response to NP, inserted into Ig H-chain expression cassettes, and transfected into J558L (H−; λ1+) myeloma cells. These transfectoma Abs expressed a remarkably wide range of measured affinities (K a= 5× 10 4-1.3× 10 6 M− 1) for NP …