Which statement is true concerning unexpected antibodies that react at 37C?

• A. They are considered clinically significant
• B. They are always clinically insignificant
• C. They react only at immediate spin
• D. They are never detected during screening

Answer: (A)

Reactivity at body temperature means the antibody binds red cells under physiologic conditions, which makes it capable of causing hemolysis in a transfusion recipient or in pregnancy. That’s why an unexpected antibody that reacts at 37°C is considered clinically significant: it has real potential to cause transfusion reactions or hemolytic disease of the fetus and newborn, and it often requires selecting antigen-negative blood or more thorough crossmatching.

This behavior also fits with how screening is done in practice—panels are tested at 37°C and with antiglobulin, so antibodies that react at this temperature are typically revealed during routine screening and identification. In contrast, antibodies that react only at immediate spin (room temperature) are usually IgM and less likely to cause clinically significant problems. So, recognizing 37°C reactivity as clinically significant helps guide safer transfusion planning and patient management.