Induction of histiocytosis and erythrophagocytosis by IL-4. (A) Hematoxylin and eosin stain (×100) of tissue from day 3 of IL-4 pump (1 μg/hour), showing activated luminal macrophages and erythrophagocytosis (arrows) within the liver. (B) Hematoxylin and eosin stain (×100) of erythrophagocytosis within the bone marrow. Immunohistochemistry (×40) for F4/80 showing increased cellular density within red pulp of IL-4 mini-pump–treated spleen (D) compared with control (C). Immunohistochemistry (×100) for F4/80 showing more diffuse and larger F4/80⁺ Kupffer cells within the liver of IL-4 mini-pump–treated mice (F) compared with controls (E). Ym1 immunohistochemistry in (G) liver and (H) spleen (×100) of IL-4 mini-pump–treated mouse.