Non-D Rh antibodies appearing after apheresis platelet transfusion: stimulation by red cells or microparticles?

Background  Apheresis platelets (APs) have gained favour over whole blood-derived platelets on the presumption that they are less likely to provoke alloimmunization to red-blood-cell antigens. Case Reports  Non-D Rh antibodies appeared in three patients after apheresis platelet transfusion. Anti-C and anti-E arose in two female patients with previous antigen exposure. Both anti-c and anti-E arose in a male recipient with no prior transfusion history. Materials and Methods  Fifty APs were analysed for residual RBCs and RBC-derived microparticles, using samples obtained from a local blood centre. Cells and microparticles were quantified with a flow cytometry gating scheme, using PE-labelled anti-CD235a (glycophorin A) and FITC-labelled anti-CD41a (platelet gp IIb/IIIa) to distinguish lineage. Results  Apheresis platelets were found to contain a mean of 7·5 × 106 (95% C.I. [6·3–8·5 × 106]) RBCs on one manufacturer’s device and 5·2 × 106 (95% C.I. [4·0–6·3 × 106]) RBCs on another’s. RBC-derived microparticles averaged 210·7 × 106 (95% C.I. [166·2–254·2 × 106]) on one manufacturer’s device and 232·3 × 106 (95% C.I. [194·3–272·9 × 106]) on another’s. These counts all correspond to volumes of < 1 μl. Conclusion  Despite RBC contamination of APs below commonly accepted thresholds for Rh immunogenicity, AP transfusion can provoke non-D Rh antibody formation. RBC-derived microparticles, smaller but more numerous than RBCs, are volumetrically comparable and may be a hitherto underappreciated antibody stimulus. Further microparticle research will guide considerations of extended phenotypic matching of platelet components.