The Presence and Persistence of Pregnancy-Associated Red Blood Cell Alloantibodies in Blood Donors

Background: Females have higher RBC alloantibody prevalence than males, presumably due to exposure to non-self RBC antigens through pregnancy as well as transfusion. Given the lack of routinely available and longitudinal data on lifelong pregnancy and transfusion histories, few studies have been able to distinguish pregnancy versus transfusion as contributing risk factors for RBC alloantibody induction or for the persistence of RBC alloantibody detectability. We hypothesized that pregnancy would be an important source of persistently detected RBC alloantibodies and investigated this in a longitudinal blood donor database.

Study Design and Methods: Donor/donation data were obtained from 4 US blood centers from 2012-2016. RBC antibody screen results and antibody identification (using tube, gel, or solid phase), donor demographics, pregnancy history, and transfusion history were evaluated. Blood donors were included in this analysis if they had an RBC alloantibody detected at any given donation as long as they had at least one subsequent blood donation and antibody screen. Anti-D antibodies detected, but present for < 6 months in females <50 years old were presumed due to passive RhIg and were excluded from this analysis. RBC alloantibody persistence was defined as an RBC alloantibody detected at each subsequent donation following its initial identification and RBC alloantibody evanescence was defined as one or more subsequent donations without re-detection of the alloantibody. Rates of donor alloantibody persistence were calculated as blood donors with persistent alloantibodies divided by blood donors with any alloantibody, and patterns of persistence were characterized by donor sex and presumed alloantibody source exposure.

Results: 503 blood donors had a detectable RBC alloantibody and a follow-up antibody screen; 374 (74%) were female and 130 (26%) were male. Of the 374 alloimmunized females, 210 (56%) reported prior pregnancy and no transfusion, 124 (33%) reported prior pregnancy and transfusion, 26 (7%) reported no pregnancy or transfusion, and 12 (3%) reported prior transfusion and no pregnancy. 215/334 (64%) of previously pregnant females had RBC alloantibodies that remained detectable throughout the duration of the study. The mean duration of detectability for persistent antibodies in previously pregnant females was 640 days, compared to a mean time to disappearance of 183 days for evanescent antibodies. In contrast, males and all never-pregnant females were less likely to have persistently detectable antibodies, with 52% (87/167) being in this category (chi square p = 0.008). The mean duration of detectability for persistent antibodies in males and never-pregnant females was 576 days, compared to a mean time to disappearance of 290 days for evanescent antibodies. Previously pregnant females with a reported history of prior transfusion (n=124) had the highest rate (77%) of RBC alloantibody persistence. The alloantibody specificities of previously pregnant females included E (96), D (81), K (78), Fy (21), C (47), MNS (38), and Jk (18). Of these, Fy and non-passive D were most likely to be persistent (each 86%), followed by C (79%), K (74%), E (59%), Jk (55%), and MNS (34%).

Conclusion: Data from this multi-center blood donor database, with documented life-long pregnancy and transfusion histories, highlight the role that pregnancy plays in RBC alloimmunization. Importantly, pregnancy was an exposure source in almost all studied RBC alloimmunized female healthy blood donors and was the sole reported exposure in the majority of donors. Although these data cannot definitively identify the source of non-self blood group antigen exposure in blood donors with both pregnancy and transfusion histories, they show that antibodies in previously pregnant females are long lasting. It is possible that length of exposure to non-self RBC antigens, the relatively young age of females during childbearing, long standing fetal white blood cell microchimerism in the maternal circulation, or other variables may impact plasma cell longevity and/or antibody production, and thus the persistence of pregnancy (versus transfusion) associated RBC alloantibodies. Given the numbers and persistence of such RBC alloantibodies, we recommend that pregnancy history be taken into consideration in all RBC alloimmunization studies involving females at or beyond childbearing age.