The fluorescence activated cell sorter was used to determine the class of immunoglobulin on the thoracic duct lymphocytes (TDL) which carried IgG memory. Although only about 3% of all TDL carried membrane IgG these cells accounted for most, if not all, of the adoptive IgG anti-DNP response. It is concluded that both CR+ and CR- cells mediating IgG memory in rat TDL bear the same class of membrane immunoglobulin as that secreted by their differentiated progeny. The class of membrane immunoglobulin on CR+ and CR- rat TDL was also examined. It was found that IgM+ cells, which made up over 80% of all Ig+ cells, were virtually all CR+. In contrast, the few percent of IgG+ and IgA+ cells present were to be found in both subpopulations. There was no evidence of a large population of B cells bearing exclusively heavy chains other than IgA, IgG, of IgM. The observation that some IgG+ cells as well as IgM+ cells possess a receptor for C3 appears to rule out the hypothesis that this receptor is involved in blocking a switch from IgM to IgG synthesis.

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