FL B cells express receptors for APRIL and exhibit APRIL binding. (A) Karpas, DOHH2, FL patient specimens (n = 9), and CD19⁺ B cells from healthy donors (n = 3) were stained with PE-conjugated anti-TACI (thick line), anti-BCMA (dashed line), or isotype control (solid gray line), and analyzed by flow cytometry. A representative patient histogram is shown. (B) Karpas, DOHH2, FL patient specimens (n = 5), and CD19⁺ B cells from healthy donors (n = 3) were left untreated or pretreated with heparin and then incubated with biotinylated APRIL and analyzed by flow cytometry: isotype-control (solid gray), APRIL alone (thick line), or APRIL plus heparin (dashed line).