IFNγ is intimately associated with the development of HLH-like pathology in LCMV-infected pfp^–/– mice. (A) Serum IFNγ levels were assayed in wild-type, pfp^–/– RAG^–/–, and β2m^–/– mice at various times after LCMV infection. (B) Twelve days after LCMV infection, tissues were analyzed by flow cytometry for the relative percent of macrophage-lineage (CD68⁺) cells in wild-type, pfp^–/–, and pfp^–/– mice that had received anti-IFNγ antibody. Peripheral blood (C) neutrophil, (D) platelet, and (E) hemoglobin levels were assayed in the same groups. Data (± standard error) are representative of at least 2 experiments with at least 3 mice in each group.