CP Biology – Inheritance Practice Test 2026 - Free Practice Questions and Study Guide

The key idea is that transfusion compatibility depends on surface antigens: A, B, and the Rh factor. O negative lacks all three (no A, no B, no Rh), so its red cells don’t display A, B, or D antigens. Because recipients won’t have antibodies against those antigens, O negative can be donated to anyone safely, making it the universal donor. For the other side, AB positive has both A and B antigens and the Rh antigen, but no anti-A or anti-B antibodies in the plasma, so someone with AB positive can receive blood from any ABO type and Rh status. That’s why AB positive is the universal recipient.