Summary: ABO(H) blood group antigens and corresponding anti-ABO(H) alloantibodies were discovered over a century ago as the first polymorphisms in the human population and continue to be the most common immunological barrier to transfusion and transplantation. However, despite the fundamental nature of this discovery, very little is surprisingly known regarding the factors that govern anti-ABO(H) antibody formation or the fine details of the ABO(H) alloantigen targets responsible for hemolytic transfusion reactions (HTRs). As a result, current approaches designed to detect ABO(H) antigens and anti-ABO(H) antibodies largely rely on the same agglutination strategy leveraged by Landsteiner over 122 years ago. While ABO(H) incompatible RBC transfusion can result in a hemolytic transfusion reaction (HTR), only half of patients who receive ABO(H) incompatible RBCs experience this outcome. However, the factors that contribute to variable ABO(H) incompatible HTRs remain largely unknown. This limitation in our understanding is a direct consequence of the complexity of the post-translational modifications that comprise ABO(H) antigens and a historical limitation in the tools needed to study both ABO(H) antigens and the antibodies that develop against them. While a variety of highly novel tools have begun to revolutionize the field of glycosciences – the study of carbohydrate modifications – these tools have not been as thoroughly applied to perhaps the most common and arguably most clinically significant carbohydrate structures within the human population – ABO(H) blood group antigens. Fundamental questions surrounding anti-ABO(H) antibodies and their target antigens often require disparate areas of expertise, including glycosciences, immunology, hematology and microbiology, which has directly limited the study of this foundational discovery in transfusion medicine. To overcome this challenge, we have assembled a highly collaborative and integrated team of physicians and scientists with expertise in these fields. The combined collaborative history of the Project leaders (>50 papers and >15 years of collaboration) and the use of newly developed models and tools specifically designed to define factors that govern anti-ABO(H) antibody development and the specific ABO(H) targets on RBCs responsible for HTRs. In doing so, this program project grant (PPG) will provide a unique opportunity to define fundamental features of anti-ABO(H) antibody development and the ABO(H) targets that result in HTRs that have remained incompletely understood for over a century. This will be accomplished through 3 distinct Projects and the support of complementary Cores. Project 1: Examining the impact of microbial dynamics on B cells responsible for anti-blood group antibody formation (Leader: Stowell). Project 2: Convergence of innate immunity and microbial communities in the regulation of anti-blood group antibody development (Leader: Arthur). Project 3. Defining the distinct ant...