Not all blood types are compatible with each other. But do you know why? Red blood cells have tiny markers on their surface called antigens. Your immune system is trained from birth to recognize which antigens belong in your body. If someone receives blood with antigens their body doesn’t recognize, the immune system treats those cells like an invading threat. It immediately attacks them. The red blood cells start clumping together and breaking apart in the bloodstream. This can block blood flow, damage organs like the kidneys, and trigger a severe reaction that can become life-threatening. That’s why blood types have to be matched carefully before a transfusion. It’s your immune system deciding what belongs and what doesn’t.