Complement activation in response to PF4/heparin complexes among healthy donors defines a donor phenotype. (A) Plasma from healthy donors (n = 10) was incubated with buffer, PF4 ± heparin, or heparin alone, and binding of C3c to PF4/heparin complexes was measured by antigen-C3c capture ELISA assay. The graph shows the anti-C3c absorbance of donor plasma incubated with different antigens. Each symbol represents an individual donor. Results are shown from a representative experiment performed a minimum of 3 times, with multiple donors in each experiment. (B) Complement activation by PF4/heparin of donors (1, 2, 3, and 4) was determined during a period of 626 days (∼1.7 years) and normalized to donor 1, who was studied at all points. The graph shows percentage PF4/heparin complement activation relative to donor 1 on different days. **P < .0001.