LAUSR

Various authors have reported on the potential for Rh alloimmunization following transfusion of RhD-positive platelets to RhD-negative recipients. The risk of alloimmunization is greater for recipients of whole blood-derived (WBD) pooled platelets compared to apheresis platelets, as pooled platelets generally contain more than 100-times as many residual red blood cells (RBCs) as apheresis units. Despite a greater incidence, the risk from pooled platelets is minimal, and it is controversial as to whether Rh immune globulin (RhIg) should be administered to RhD-negative recipients of RhD-positive platelet units. Those that recommend administration do so only in certain scenarios and/or for specific patient populations. Given the rarity of RhD-alloimmunization following platelet transfusion, and apheresis platelet units in particular, we report a case of alloimmunization following transfusion of one unit of apheresis platelets. A 91-yearold female with newly diagnosed myelodysplastic syndrome/myeloproliferative neoplasm was found to be thrombocytopenic. Eltrombopag was initiated, and blood typing as well as antibody detection testing (screen) were performed. The patient was determined to be ABO type A RhD-negative with a negative antibody screen via column agglutination technology (gel). The patient reported she had three prior pregnancies. RhIg was not available at the time of her first two pregnancies, and Rh status of those offspring is unknown to the patient. The offspring of the third pregnancy was RhD-negative and RhIg was not indicated nor given. She denied any history of transfusion or alloantibody identified during any pregnancy or previous laboratory evaluation. She was administered 1 unit (342 ml) of type A RhD-positive, pathogen-reduced apheresis platelets in platelet additive solution (PAS) given her thrombocytopenia. Two weeks later, a repeat antibody screen was negative in gel, and the patient was administered one unit of type A RhD-negative RBCs. The patient presented 6 months later, and a repeat antibody screen was positive in gel (reactivity: 2+ in one screening cell, 3+ in one screening cell); both anti-D and anti-C alloantibodies were identified (2–3+ reactivity in gel). Per report from both the patient and the hematology team, the patient had received no blood products nor RhIg in the interim. Review of Care Everywhere medical charts revealed the patient had not received care outside of our facility, and there were no documented outside transfusions. A serologic phenotype revealed the patient did not express the C antigen. This case is notable as it highlights the potential for one unit of apheresis platelets stored in PAS to induce alloimmunization. Importantly, we cannot definitively conclude whether this case reflects primary immunization, or rather induction of a memory response. While we have no evidence that the patient had previously been alloimmunized, she did endorse three pregnancies, of which two fetuses were of unknown RhD status and no RhIg was administered. Should the patient have experienced alloimmunization through one or more of her pregnancies or an unreported/unknown transfusion, resulting in formation of anti-D and/or -C (as well as potentially anti-G), which subsequently evanesced, then the patient's immune system may have been restimulated as a result of low-level exposure to Rh antigens. Alternatively, the platelet transfusion may indeed have been the first exposure to both the Dand C-antigens and resulted in primary alloimmunization, with antibodies detected several weeks post-exposure. Although we cannot conclude with certainty which pathway led to detectable anti-D and -C (and/or potentially anti-G) in our patient, this case clearly highlights the potential for a single apheresis platelet unit to contain a sufficient number of RBCs to elicit an immune response to Rh antigens, either primary or secondary. While there was no concern for the potential development of hemolytic disease of the fetus and newborn in this patient, this case may provide support for those who continue to administer RhIg following platelet transfusion in women of childbearing age as primary immunization cannot be excluded. Furthermore, this case illustrates that immune responses to RBC antigens may be preserved in a subset of individuals in whom immunosenescence would otherwise be expected based on their age, and highlights the importance of further investigation into why certain individuals appear to be more susceptible to alloantibody formation. Received: 18 November 2022 Revised: 3 January 2023 Accepted: 11 January 2023