Stimulation of B lymphocytes via HLA class II increases sensitivity to Fas-induced apoptosis. Splenic B lymphocytes (0.5 × 10⁶) were incubated for 24 or 96 hours with either L227 XL, biotinylated TSST-1 cross-linked with streptavidin, or PMA ionomycin. 1 μg/mL anti-Fas MoAb was added 24 hours before treatment of the cells for FACS analysis. Bars correspond to the standard error for each time point.