Concept:The ABO blood group system classifies blood based on the presence of antigens A and B on red blood cells and corresponding antibodies in plasma. The universal acceptor is the blood group that lacks antibodies against A or B, allowing it to receive blood from all other groups.Explanation:Blood group AB has both A and B antigens on its red blood cells and has no anti‑A or anti‑B antibodies in the plasma. This absence of antibodies means AB blood can accept red blood cells from donors of type A, B, AB, and O without an immune reaction. In contrast, blood group A has anti‑B antibodies, group B has anti‑A antibodies, and group O has both anti‑A and anti‑B antibodies, limiting their ability to receive from other groups. Therefore, AB is called the universal acceptor.Answer:AB