C. parvum ELISA results (mean + 1 SD) for CD40−/− C57BL/6 mice infected with 106 C. parvum oocysts. ▴, CD40−/− mice, unmanipulated; ▵, CD40−/− mice transplanted with C57BL/6+/− bone marrow; ■, CD40−/− mice transplanted with CD40−/− marrow; □, CD40+/− mice transplanted with CD40−/− marrow. OD, optical density.