Effect of IFN-γ on its own expression in B cells. Purified primary B cells were activated with IFN-γ for up to 72 hours, then mRNA was extracted and reverse transcribed. cDNA was amplified with IFN-γ primers. (A) PCR bands for the 48-hour time point. (B) IFN-γ mRNA expression was quantified as described. Results (means ± SEMs) are expressed as -fold increases in normalized IFN-γ cDNA values versus the level at the 1-hour time point.