 In this study, blood samples will be collected from injured individuals who arrive at the University of California at Davis Medical Center. Individuals who receive a blood transfusion, as well as a control group of individuals who do not receive a transfusion, will be approached to enroll in the study. Individuals who agree to participate will have their blood collected at Days 7, 14, and 28 and Months 3 and 6. Study researchers will analyze the blood for evidence of microchimerism. Participants with microchimerism will undergo additional blood collection at Months 12, 18, 24, 30, and 36. Information will be collected from all participants on health status, injury characteristics, hospital care, blood transfusion details, and blood donor characteristics. 