High concentrations of WM-derived IgM impair platelet spreading, aggregation, and thrombin generation. HD WP spreading (n = 4-5) for 20-60 minutes on (A,C-F) fibrinogen or (B,G-J) collagen, then visualized for actin (red) and α-tubulin (green), in the presence of (C-F,G-J) vehicle (blue), BSA (equimolar to IgM, 4.11 mg/mL, pink), or IgM (60 mg/mL, red). (C,G) Number of adhered platelets; (D,H) platelet surface area; (E,I) circularity; or (F,J) aspect ratio, average of 5 field of views. (K) Representative aggregation trace, mean of duplicates. (L) WP aggregation responses to collagen (5 μg/mL), TRAP-6 (10 μM), and ADP (5 μM). (M) Representative thrombogram, mean of duplicates. Thrombin generation in HD samples (n = 8), with (N,R) lag time, (O,S) ETP, (P,T) peak thrombin generated, and (Q,U) thrombin generation velocity in PPP for panels N-Q or PPP + autologous WPs for panels R-U. For panel T, mean ± SD. P values were determined by 2-way ANOVA with multiple comparisons for panels C-F,G-J,L or Friedman test for panels N-U. AUC, area under the curve.