Immunoperoxidase labeling of a paraffin-embedded lymph node section from a case of ALCL. (a) Low-power view showing areas of tumor infiltration. LF, a residual lymphoid follicle surrounded by malignant cells. The lack of staining of normal cells allows scattered malignant cells to be easily detected in (b) lymphatic sinuses or as (c) isolated cells. (d) At higher power, antibody ALK1 labels both the cytoplasm and, more variably, the nuclei of the malignant cells.