Red blood cell (RBC) alloimmunization is a leading cause of morbidity and mortality in transfusion and pregnancy settings, resulting in hemolytic reactions and other adverse sequelae. Alloimmunization occurs in up to 40% of some patient populations, including those with sickle cell disease (SCD); rates in patients with myelodysplastic syndrome, thalassemia major, and autoimmunity are also high. Most studies of RBC alloimmunization to date have focused on incidence/prevalence rates and recipient characteristics/diagnoses, and few studies have evaluated blood donor or blood unit contributions to RBC alloantibody formation. The National Heart, Lung, and Blood Institute (NHBLI) funded REDS-III vein-to-vein data set, available for public use through BioLINCC, allows a unique opportunity to evaluate RBC alloantibodies in diverse recipients across multiple institutions and to investigate potential blood donor and unit variables involved in transfusion associated alloantibody formation. This data set contains inpatient and outpatient electronic health information from four major blood centers and 12 community and academic hospitals in the United States, over a 4-year time-period from 2013-2017. A query of the data set has shown that there are 651 subjects with a new RBC antibody formed in the time interval (15 days to 16 weeks) following transfusion over which most antibodies would be expected to form. The innovation of the current proposal lies in the unique ability to link blood donor and blood unit characteristics to the recipient outcome of alloimmunization following RBC transfusion. Murine studies have shown that more rapid RBC clearance following transfusion is associated with a higher degree of RBC alloimmunization, regardless of whether that increased clearance is due to longer storage duration or other causes. As such, we hypothesize that factors associated with poorer RBC storage in humans, including red cell storage duration, donor sex, or irradiation, will impact the likelihood of alloantibody formation. We propose to complete a case: control (1:4) study, matching cases and controls for age, sex, diagnosis, race, and ethnicity. With this study design, we propose: Aim 1) To investigate blood donor characteristics associated with RBC alloimmunization in transfusion recipients and Aim 2) To investigate RBC unit characteristics associated with RBC alloimmunization in transfusion recipients. Long term, any blood donor or RBC unit characteristic associated with alloimmunization could be modified for RBC units intended for patients at highest risk of RBC alloimmunization. If new antibody formation could be minimized, transfusion safety will increase.