Mice transgenic for IL-4 expression have liver erythrophagocytosis, splenic histiocytosis, and extramedullary hematopoiesis. (A) Hematoxylin and eosin stains of B6 WT spleen (left) and spleen (×4) from IL-4 TG.UG mice (right) showing areas of normal white pulp (arrows). Histiocytosis and extramedullary hematopoiesis are only seen in the TG.UG spleen as shown in areas marked with yellow stars. (Insets) Original magnification ×100 (view of red pulp). Stars in wild-type mice indicate normal red pulp. (B) Hematoxylin and eosin stain (×40) of liver showing erythrophagocytosis (arrows). (Inset) Original magnification ×100 from a representative 11-month-old B6 IL-4 transgenic mouse. (C) Bone marrow cellularity from 1 long bone of IL-4 transgenic and age-matched wild-type B6 mice (4 from each group evaluated).