Case report of a 74-year-old woman (with a history of Parkinson's disease) who experienced an acute hemolytic transfusion reaction caused by anti‑CD99 alloantibody, with laboratory evidence linking donor CD99 expression to red cell phagocytosis.

This has minimal direct value for Parkinson's therapeutic discovery but is clinically relevant for transfusion safety in PD patients and highlights a rare immune complication rather than informing PD mechanisms or drug targets.

BACKGROUND: Anti-CD99 is a rare alloantibody with unclear clinical significance in transfusion medicine. We report a case of acute hemolytic transfusion reaction associated with anti-CD99 in a female patient. CASE REPORT: The patient was a 74-year-old woman originally from Kyushu Prefecture with a history of pregnancy and transfusions. The patient was also diagnosed with Parkinson's disease 13 years ago and recently hospitalized with intestinal obstruction and aspiration pneumonia. During hospitalization, she developed anemia requiring red blood cell transfusions; pre-transfusion antibody screening before the second transfusion revealed pan-reactivity with all panel cells. Antibody identification testing at the Japanese Red Cross Kyushu Block Blood Center detected anti-CD99 in the patient's serum. As CD99-negative blood was unavailable, two units of randomly selected red blood cells derived from male donors were transfused. Shortly after the second transfusion, the patient developed chills, fever, hematuria, and laboratory evidence of hemolysis. Flow cytometry and monocyte phagocytosis assays demonstrated that CD99 expression on donor red blood cells correlated with susceptibility to phagocytosis. The patient subsequently died on day 12 after the second transfusion due to worsening pneumonia. CONCLUSION: Antibody-producing CD99-negative individuals are extremely rare; in Japan, only 10 cases have been identified since 1993 at Japanese Red Cross blood centers. All of these patients were women from the Kyushu region. The present case demonstrates the potential of anti-CD99 to cause acute hemolytic transfusion reactions and represents a valuable clinical report, emphasizing the importance of careful transfusion management in patients with anti-CD99.