What is the estimated percentage of patients immunized to blood group antigens?

• A. 0.1%-0.5%
• B. 1%-4%
• C. 5%-10%
• D. 10%-20%

Answer: (B)

Alloimmunization to red cell antigens happens when the immune system encounters foreign blood group antigens during transfusion or pregnancy and mounts an antibody response. In the general transfusion population, this occurs at a relatively low rate, roughly a few percent, about 1–4%. That’s why this option is the best fit: it reflects the typical risk across diverse patients who are exposed to non-self antigens enough to form alloantibodies. The lower range (0.1–0.5%) would understate how often exposure leads to an antibody response, while the higher ranges (5–10% or 10–20%) describe rates seen only in subgroups with very heavy exposure (such as many prior transfusions or multiple pregnancies) rather than the general patient population.