When are patients most likely to experience hemolytic transfusion reactions?

Patients are most likely to experience hemolytic transfusion reactions after multiple transfusions without appropriate screening. In this scenario, the risk is heightened because each transfusion can expose a patient to foreign antigens present on red blood cells from the transfused blood. If a patient develops antibodies against these antigens due to previous transfusions, subsequent transfusions with incompatible blood can lead to a hemolytic reaction, where the immune system attacks the transfused red blood cells.

The importance of proper screening, such as crossmatching and testing for antibodies, is crucial in preventing these reactions. In contrast, during a first-time transfusion, the risk of an immediate hemolytic reaction is lower because the patient's immune system has not yet been sensitized to any foreign red cell antigens. While complications can arise in any of the contexts listed, the likelihood of developing a hemolytic reaction increases significantly when a patient has previously received blood transfusions without adequate screening for compatibility.