Expression of IL-9Rα and IL-9 in a reactive lymph node and in primary ALK⁺ ALCL tumors from patients. (A) IL-9Rα is strongly expressed in a significant number of cells within the germinal center (left side) and the mantle zone of a reactive lymph node. The frequency and intensity of expression of IL-9Rα are relatively diminished in the marginal zone and interfollicular areas (original magnification × 200). (B) IL-9 is strongly expressed in scattered small lymphoid cells and mast cells in a reactive lymph node. Most of these cells are localized in the interfollicular areas as well as around and within the lymph node sinuses (original magnification × 200). (C-D) An example of a lymph node showing sclerotic tissue with dense infiltration by ALK⁺ ALCL cells that are strongly positive for IL-9Rα (C) and IL-9 (D; original magnification × 200). (E-F) Another example of a lymph node involved by ALK⁺ ALCL demonstrating large neoplastic cells that are positive for IL-9Rα (E) and IL-9 (F). The large neoplastic cells are confined to the lymph node sinuses, a characteristic morphologic feature of ALK⁺ ALCL (original magnification × 200). In all of the tissue samples, the staining for both IL-9Rα and IL-9 was membranous and cytoplasmic, whereas the nuclei were negative for the 2 proteins.