Why your body might attack a life-saving blood transfusion

Science · Published 25 October 2026 Swipe more like this →

In short: In the 17th century, early attempts at blood transfusions were often fatal. Today, medical teams use complex biological markers to ensure compatibility. The secret lies in antigens—tiny molecular "nametags" on your red blood cells that tell your immune system whether a substance belongs inside you or is an invader.

The ABO blood group system, the most well-known classification of human blood, was developed by biologist Karl Landsteiner in 1901. This system categorizes individuals into four main types: A, B, AB, and O. These categories are determined by specific antigens on the surface of red blood cells (erythrocytes). For instance, Type A blood contains the A antigen, while Type B contains the B antigen; Type AB has both, and Type O has neither.

Beyond the ABO system, doctors also look for the Rh factor. This is a protein that determines if a person's blood is "positive" or "negative." If the RhD antigen is present, the blood is positive; if it is absent, it is negative. Together, these two systems create eight common blood types, such as O positive (the most common in the U.S.) and AB negative (the least common). However, the complexity of human biology goes even deeper: there are over 600 other antigens that can attach to red blood cells.

Understanding these markers is a matter of survival because of how the immune system functions. An antigen acts as a marker; if your immune system encounters an antigen it doesn't recognize as "self," it may trigger a reaction. This was tragically demonstrated in 1667 when Richard Lower transfused lamb blood into a human, and later by Jean-Baptiste Denis, whose experiments led to the first recorded hemolytic transfusion reactions—where red cells clump and burst. Modern medicine prevents these reactions by ensuring that donated blood is compatible with the recipient's specific antigen profile.

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