Reporting a rare, clinically significant, naturally occurring anti-Cw alloantibody in a whole blood donor: A kingpin in vein-to-vein transfusion chain.

Anti-Cw (C Willis) is a low-frequency immunoglobulin against red cell antigen Cw in the Rhesus (Rh) blood group system. Anti-Cw can either be naturally occurring or develop as a result of red blood cell sensitization through transfusion or during pregnancy, and is rarely associated with clinically significant hemolytic disease of the fetus and newborn. However, anti-Cw, being a low-frequency antigen, is not always included in the commercial antibody screening panels and also not phenotyped for in-house pooled red cells used for single-cell indirect antiglobulin testing for blood donors in various blood centers. Here, we report a case of a whole blood donor who was identified with an anti-Cw antibody.