In chronically transfused patients, the risk of developing red cell alloimmunization increases by what range?

• A. 20% - 50%
• B. 2% - 8%
• C. 1% - 4%
• D. 5% - 10%

Answer: (B)

Repeated exposure to foreign red cell antigens during ongoing transfusion can sensitize the immune system and lead to alloimmunization. As transfusion exposure accumulates, the chance that a recipient will form alloantibodies against non-self antigens rises to a few percent overall. In chronically transfused patients, this cumulative risk is typically cited as about 2% to 8%.

This range reflects how antigen differences between donor and recipient, the number of transfusions, and the patient’s underlying condition (for example, sickle cell disease or thalassemia) influence likelihood of antibody formation. Extended antigen matching (beyond the basic ABO/Rh) can reduce this risk, but without such matching the general estimate fits within the 2–8% window.

The other ranges are either too high for the average chronically transfused population (20–50%) or too narrow (1–4% or 5–10%) to capture the common variability seen across different patients and transfusion histories.