DDR1 overexpressing primary HRS cells intimately associate with collagen. (A) Immunohistochemistry stain for DDR1 expression in Hodgkin lymphoma. DDR1 was either absent or only weakly expressed in normal GC cells of a nonmalignant tonsil (left, upper panel). Original magnification ×200. Remaining panels show representative examples of DDR1 expression in primary HRS cells. DDR1 expression in HRS cells was localized to the cell membrane (arrows) or as focal aggregates in the cytoplasm (broken arrows). Original magnifications: right upper panel ×400; left lower panel ×600 oil immersion lens; right lower panel ×1000 oil immersion lens. (B) van Gieson’s stain of a nonmalignant tonsil shows limited collagen deposition within the GC (left, upper panel; original magnification ×400) and by comparison DDR1-positive HRS cells (arrows) are frequently intimately associated with collagen (red staining). Original magnifications: right upper panel ×600 oil immersion lens; lower panels ×1000 oil immersion lens. GC, germinal center.