Tumor cell–specific BCR-mediated signaling is common in FL. (A) BCR-mediated signaling at 4 and 30 minutes following BCR cross-linking in the presence of H₂O₂ (α-μ/γ+ H₂O₂) was measured in subsets of the CD20⁺ B cells from 4 FL patient samples (FL-P07, FL-P08, FL-P09, FL-P11). FL B cells (CD20⁺ Bcl-2^hi nontumor light chain^–, dark arrow) and tumor-infiltrating nonmalignant B cells (CD20⁺ Bcl-2^lo nontumor light chain⁺, light arrow) were distinguished and their signaling was compared by coloring heat map squares relative to the fold induction of phosphorylation relative to the unstimulated sample (0 minute). (B) Flow cytometry analysis of FL patient biopsy cells (FL-P12) stimulated by a combination of BCR cross-linking and H₂O₂ (α-μ/γ+ H₂O₂) for various times (4, 30, 60, or 90 minutes) or left unstimulated (0 minute). BCR-mediated phosphorylation of Erk1/2 and p38 was measured in nonmalignant B cells (CD20⁺ λ isotype, light arrow) and compared with that in FL B cells (CD20⁺ Bcl-2⁺ λ^–, dark arrow).