Biological Systems MCAT Practice Exam 2025 – Complete Study Resource

The designation of blood types, particularly concerning blood donation, is based on the presence or absence of specific antigens on the surface of red blood cells. Type O blood is recognized as a universal donor because it lacks both A and B antigens. This absence means that it does not provoke an immune response in recipients who have any of the other blood types (A, B, AB, or O), allowing it to be safely transfused to individuals with various blood types.

Moreover, O- (O negative) is particularly significant because it not only lacks the A and B antigens but also does not have the Rh factor (the D antigen). This makes O- blood compatible with any Rh type, reinforcing its status as the universal donor. Patients who receive transfusions are at risk of transfusion reactions when their blood contains antigens that interact with the donor’s blood components, which is avoided with O- donations.

The other blood types listed (AB+, A+, and B-) each contain specific antigens that could lead to a risk of hemolytic reactions if transfused into incompatible recipients. Thus, O- is essential in emergency situations where blood types need to be matched quickly, as it can be safely given to anyone without immediate compatibility testing.