Very low affinity B cells form germinal centers, become memory B cells, and participate in secondary immune responses when higher affinity competition is reduced.

To understand the relationship between the affinity of the B cell antigen receptor (BCR) and the immune response to antigen, two lines of immunoglobulin H chain transgenic (Tg) mice were created. H50Gmu(a) and T1(V23)mu(a) mice express mu H chain transgenes that associate with the lambda1 L chains to bind the (4-hydroxy-3-nitrophenyl)acetyl hapten with association constants (K(a)s) of only 1.2 x 10(5) M(-1) and 3 x 10(4) M(-1), respectively. Both lines mounted substantial antibody-forming cell (AFC) and germinal center (GC) responses. H50Gmu(a) Tg mice also generated memory B cells. T1(V23)mu(a) B cells formed AFC and GCs, but were largely replaced in late GCs by antigen-specific cells that express endogenous BCRs. Thus, B lymphocytes carrying BCRs with affinities previously thought to be irrelevant in specific immune responses are in fact capable of complete T cell-dependent immune responses when relieved of substantial competition from other B cells. The failure to observe such B cells normally in late primary responses and in memory B cell populations is the result of competition, rather than an intrinsic inability of low affinity B cells.