ABO blood system

The ABO blood system classifies human blood based on the presence or absence of specific antigens—molecular markers—on red blood cells. There are four main blood types: A, B, AB, and O. Type A has A antigens, B has B antigens, AB has both, and O has neither. The immune system can produce antibodies against the antigens not present on your own red blood cells. Compatibility between donor and recipient blood is crucial for transfusions; mismatched blood can cause dangerous reactions. This system helps ensure safe blood transfusions and is important in pregnancy management.