Jajosky, Ryan P.

Abstract Background Babesia and Plasmodium are intraerythrocytic parasites that cause babesiosis and malaria, respectively. Significant associations between human blood group antigens and malaria are known; however, few studies have examined the relationship between blood group antigens and babesiosis. Study Design and Methods From June to August 2025, 100 Babesia polymerase chain reaction (PCR)− and 100 B. microti PCR+ human patient samples tested at a national reference laboratory underwent ABO and D phenotyping and red blood cell (RBC) genotyping of 37 antigens. Results The predicted phenotype of S‐s+ was found in 32% and 50% of Babesia − and B. microti + patients, respectively (χ 2 = 5.97, p = .015, odds ratio = 2.125, 95% confidence interval 1.18–3.78). Two B. microti + patients had the predicted phenotype of Fy(a‐b‐), meaning that Duffy is unlikely to be an essential invasion receptor. There were no statistically significant differences in ABO or D expression between the groups. Discussion Because multiple testing corrections were not applied to the statistical analyses, the S/s predicted phenotype results must be viewed as hypothesis‐generating. This may be the first study to link B. microti infection to human RBC S/s antigens, which are known to impact B. divergens and P. falciparum . Additional studies are needed to understand the impact of the babesiosis patient's blood type on B. microti infection and whether the transfusion of RBCs expressing specific antigens can impact the disease.