Erythroblasts undergoing apoptosis. TER119⁺ erythroblasts were enriched by magnetic cell sorting system with antimouse TER119 MicroBeads purchased from Miltenyi Biotec. Apoptosis in erythroblasts from naive mice (A), naive mice treated with EPO (B), infected mice (C), and infected mice treated with EPO (D) was detected using the In Situ Cell Death Detection Kit–FITC (TUNEL assay) from Boehringer Mannheim–Roche Diagnostics. Numbers represent frequencies of TUNEL⁺ splenocytes. Data are means ± SEM of 3 mice per group except for panel A, where cells were pooled from 4 mice. When 2 sets of data are labeled with superscripts of different lowercase letters, it indicates that these 2 sets of data are statistically different (P < .05). Similar results were obtained in 2 independent experiments.