Immunoglobulin classes of antibody-forming cells in mice. I. Localized hemolysis-in-agar plaque-forming cells belonging to five immunoglobulin classes.

Spleen cell suspensions from mice that had been immunized with an optimal dose of sheep erythrocytes were assayed by a replicate plating technique capable of detecting and separately differentiating plaque-forming cells (PFC) producing IgM and/or γ1 hemolysin. The mice were tested 5 1/2 days after immunization when γ1 PFC were first detectable in significant numbers; approximately equal to the number of IgM PFC. Only 2% to 4% of the PFC examined could be considered as producing IgM and γ1 hemolysin simultaneously. These results are inconsistent with the theory that the rapid appearance of a large number of IgG-PFC between day 4 and day 51/2 is the result of antibody-producing cells switching from IgM to IgG production.