IL-4–treated mice develop weight loss, signs of HLH, and evidence of accumulation of alternatively activated macrophages. (A) Wild-type B6 mice were treated with 1 μg/hour IL-4 mini-pumps or PBS control (5 in each group) for 3 days. The indicated serum and blood cell indices were measured at that time. This experiment was repeated with similar results. (B) Mononuclear cells were isolated from the livers of these mice, enumerated and stained for F4/80 to determine liver macrophage burden (shown are total macrophages/liver), and real-time PCR was performed to evaluate markers of classic and alternative activation (expression levels of untreated mice set at 1; shown are fold increases of expression of the indicated gene over control by IL-4–treated mice).