Antibodies of the secondary response can be expressed without switch recombination in normal mouse B cells.

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Abstract
A few percent of mouse splenocytes express isotypes characteristics of the secondary response together with IgM, and some cells express these isotypes alone. Populations of small memory cells were isolated that express; IgM but not IgG1; IgM but not IgA; IgM and IgG1; IgM and IgA; and IgG1 but not IgM. Their DNA was analyzed to show that there has been no switch recombination or deletion in the Ig constant region (C) genes. Sandwich RNA hybridizations demonstrated that cells expressing IgG1 contain nuclear RNA that have both C.mu. and C.gamma.1 sequences, and that cells expressing IgA contain nuclear RNA that have both C.mu. and C.alpha. sequences. The expression of an isotype characteristic of the secondary response in memory cells is apparently accomplished by alternative RNA processing of large (up to 180 kilobases) nuclear RNA transcripts that span the H chain gene locus.