**Core Concept**
The ABO blood group system is the most immunogenic RBC blood group system, meaning it can trigger a strong immune response and cause hemolysis when incompatible blood is transfused. This is due to the presence of antibodies against the A and B antigens in individuals who do not possess these antigens on their RBCs.

**Why the Correct Answer is Right**
The ABO blood group system is the most immunogenic because it involves the expression of A and B antigens on RBCs, which are recognized by the immune system as foreign substances. Individuals who possess the A or B antigen on their RBCs will produce antibodies against the other antigen, leading to a strong immune response. This is mediated by the action of the enzyme galactosyltransferase, which is involved in the synthesis of the A and B antigens.

**Why Each Wrong Option is Incorrect**
**Option A:** The Kell blood group system is also immunogenic, but it is less so than the ABO system.
**Option B:** The Duffy blood group system is not typically considered immunogenic.
**Option C:** The Rh blood group system can also trigger an immune response, but it is generally less severe than the ABO system.

**Clinical Pearl / High-Yield Fact**
The ABO blood group system is the most important blood group system in transfusion medicine, and compatibility testing is critical to prevent hemolysis and other complications. Always check the ABO and Rh blood types before transfusing blood.

**Correct Answer:** C. Kell blood group system