Immunostaining of classical HL for ligands of aberrantly expressed RTKs. Immunostaining of a lymph node with reactive lymphoid hyperplasia (A) and of a classical HL (B) for collagen type 1, the high-affinity ligand for DDR2, using alkaline phosphatase. Although only the capsule and occasional vessels are stained in lymphadenitis, sclerotic bands containing collagen type 1 and branching into the tumor infiltrate are numerous in HL. Immunostaining of a lymph node in a patient with reactive hyperplasia (C) and classical HL (D) for NGF, the TRKA ligand. NGF-positive cells were rarely observed in the reactive lymph node, whereas such cells were frequent in the tumor infiltrate. (E) Immunostaining of a classical HL for PDGFA, a high-affinity ligand for PDGFRA. PDGFA is expressed by HRS cells in a patient also expressing PDGFRA. (F) Immunostaining in a patient with classical HL for EphrinB1, a ligand for EPHB1, found to be expressed by HRS cells of several patients expressing EPHB1. All magnifications are 100×. Inserts in panels A and B are 0.5×; those in panels C-F are 400×.