Naturally Occurring Anti- Jk(a): Expanding the Evidence Beyond Sensitization

Introduction: Red blood cell (RBC) alloantibodies typically develop following immune sensitization through transfusion or pregnancy. Naturally occurring antibodies, in contrast, arise without such exposure and are often directed against antigens such as ABO or Lewis. Kidd system antibodies are clinically significant, usually IgG, and rarely occur naturally. Methods: Routine pretransfusion testing was performed using automated hemagglutination (Echo® Lumena, Werfen; Norcross, GA, USA) and manual tube methods for ABO and RhD typing. Antibody screening utilized a standard three-cell panel and extended testing by solid-phase red cell adherence (SPRCA). Phenotyping was conducted manually using monoclonal anti-Jk(a) and anti-Jk(b). Comparative testing employed the indirect antiglobulin test (IAT) in tube with polyethylene glycol (PEG) enhancement. Results: Two patients were encountered with anti-Jk(a) by SPRCA despite no history of transfusion, pregnancy, or immunoglobulin therapy. Both exhibited a Jk(a)-negative, Jk(b)-positive phenotype. Reactivity consistent with anti-Jk(a) and dosage effect was observed in SPRCA testing, while IAT with PEG enhancement failed to detect the antibody. Auto control and direct antiglobulin tests were negative, and no additional clinically significant antibodies were identified. Conclusion: These findings provide further evidence that anti-Jk(a) can arise naturally, independent of sensitizing events. Detection was possible through solid-phase testing, highlighting its ability to identify weak or developing antibodies that may be missed by conventional tube methods. Awareness of such naturally occurring Kidd antibodies is essential to ensure appropriate antibody identification and selection of compatible blood for transfusion.