AbstractBALB/c mice immunized with the isologous myeloma protein MOPC‐315 (M315; α, λ2) develop antibodies most of which are directed to idiotypic antigens (Id315) located in or near the CNP binding sites of M315, In order to more precisely establish the fine specificity of anti‐Id315 antibodies, and to obtain an estimate of the size of the BALB/c anti‐Id315 repertoire, we examined anti‐Id315 antibodies from individual mice by analytical isoelectric focusing followed by autoradiography.125I‐labeled probes were prepared from purified mildly reduced and alkylated M315 protein (RA315) (7 S monomer), Fab'315, Fv315, L315, H315, DNP affinity‐labeled RA315, mildly reduced and alkylated MOPC‐460 (RA460; 7 S; α,k, Fab'460, H460, and DNP affinity‐labeled RA460. We observed that (a) the majority of BALB/c antibodies elicited by immunization with RA315 are specific for idiotypic antigens in the variable region which are rendered inaccessible in the presence of DNP‐hapten; (b) distinctly separate antibodies are specific for idiotypic antigens also located in the variable regions but which are not influenced by DNP hapten; (c) about 30 of BALB/c mice develop antibodies which appear to be specific for a determinant located in the second or third, homology regions of the constant region of the α chain which is not influenced by DNP hapten, and which is also present in the same region of H460; (d) none of the antibodies elicited by M315 recognize free L315; (e) the total antibody response is quite restricted; (f) shared spectrotypes are common between individual mice; and (g) unique or infrequently shared spectrotypes do occur. These studies demonstrate, by a direct technique, the fine specificity and the clonal and molecular heterogeneity of the B cell response induced by im
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