C5 and leukemia: In support of this prediction, we first downregulated p38 MAPK in human leukemia cell lines (U937 and KG1a) by employing the specific small-molecule inhibitor SB203580, and in its presence neither cell line responded with p38 MAPK activation after stimulation with C3a, C5a, or SDF-1, and, more importantly, downregulation of p38 MAPK resulted in upregulation of HO-1 in these cells [31].