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.