Phase contrast photomicrographs of BL cell lines cultured at 37°C for 72 hours either alone (control) or with CD40L (1 μg/mL). Cell clumping was observed either after (A) the initial 72-hour treatment time or (B) after a total 10 days of culture, which included the initial 72-hour treatment. For all experiments in which cells were pretreated with reagents, cells at 5 × 10⁵/mL received an initial culture with CD40L (with or without IL-4) for time stated and were then washed free of stimulants before replating at 5 × 10⁵/mL in fresh culture medium in the absence of stimulants. The scale bar represents 50 μm.