Acquired and transient RBC CD55 deficiency (Inab phenotype) and anti‐IFC

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Abstract
BACKGROUND: Antigens of the Cromer blood group system reside on the glycoprotein CD55 (decay‐accelerating factor). The Inab phenotype is the null phenotype of this system. So far, only five propositi have been described who exhibit this phenotype, and single‐nucleotide substitutions in the CD55 gene have been found in three of them. This report describes the first example of a patient with an acquired and transient form of the Inab phenotype. CASE REPORT: A 54‐year‐old black patient was admitted to the hospital because of abdominal pain. Multiple splenic infarctions were visualized in the abdominal computerized tomography scan, and a prophylactic splenectomy was performed. The patient's serum reacted by an IAT with all donor RBCs tested. RESULTS: Serologic analysis showed that the patient had the rare Inab phenotype and that his serum contained anti‐IFC. Flow cytometry demonstrated the absence of CD55 on his RBCs, whereas lymphocytes, monocytes, granulocytes, and platelets expressed CD55, albeit at a weaker level than cells of common phenotypes. cDNA revealed no differences from the published sequences. Flow cytometry performed 12 months after splenectomy showed reappearance of the CD55 antigen; serologic tests performed after 17 months revealed that the anti‐IFC had almost disappeared and that the RBCs were again agglutinated by various Cromer antibodies. CONCLUSION: A patient with an acquired and transient form of the Inab phenotype is described, in whom the CD55 deficiency is limited to the RBCs and is associated with splenic infarctions.