What percentage of the population lacks the Kell antigen?

• A. 9%
• B. 50%
• C. 91%
• D. 100%

Answer: (C)

The Kell antigen system has two main antigens, with the K antigen present in only a minority of people and the alternative antigen (often called k) being very common. In most populations, about 9% carry the K antigen, while roughly 91% do not express it. So, the percentage of people who lack the Kell antigen is about 91%. This matters clinically because individuals who are K-negative can form anti-K antibodies if exposed to K-positive blood, which can cause hemolytic transfusion reactions or hemolytic disease of the fetus and newborn. The other options don’t fit: 50% would mean half have K, 100% would mean everyone has K, and 9% would be the percent who have K, not those who lack it.