Characterization of two different types of direct plaque-forming cells.

At least two different types of plaque-forming cells (PFC) participating in the immune response of mice to sheep red blood cells (SRBC) are identified. This is demonstrated by (a) two PFC-peaks appearing at different times in response to the same low dose of antigen; (b) the morphology of the PFC in each of the two peaks is different; (c) the PFC in each peak respond differently to the suppressive effect of anti-SRBC serum.