What Causes Blood Type Differences
Blood type is determined by specific proteins and sugars called antigens that sit on the surface of red blood cells. Different people inherit different combinations of these antigens from their parents. The main blood type system, called ABO, has three possible antigens: A, B, and neither (which is type O). A second system, called Rh factor, determines whether you have a specific protein or not, making you either positive or negative.
The ABO System Explained
The ABO blood type system divides people into four groups. Type A has A antigens, Type B has B antigens, Type AB has both A and B antigens, and Type O has neither. You inherit one antigen from each parent, so a person with Type A might have inherited A from both parents or A from one parent and O from the other. This genetic inheritance is why blood types run in families.
The Rh Factor
In addition to the ABO system, blood is either Rh positive or Rh negative based on whether red blood cells have a specific protein called the Rh antigen. About 85% of people are Rh positive, meaning they have this protein. The remaining 15% are Rh negative, lacking this protein. Like ABO type, your Rh status is inherited from your parents.
Why Blood Type Matters
Blood types matter most during blood transfusions and pregnancy. During a transfusion, the recipient's immune system attacks blood that has different antigens on it. For example, Type A blood has antibodies against Type B blood. This is why doctors must match blood types carefully. In pregnancy, if the mother is Rh negative and the baby is Rh positive, the mother's body might attack the baby's blood cells, which doctors can now prevent with special treatment.
Genetic Inheritance of Blood Types
Each person inherits two genes for ABO blood type and one gene for Rh factor. The ABO genes follow specific patterns: A and B are dominant over O, meaning Type AB shows both antigens, while Type O only appears when you inherit O from both parents. For Rh factor, positive is dominant over negative, so you only have Rh negative blood if you inherit the negative gene from both parents. This genetic system explains why two brown-eyed parents can have a red-haired child, or why blood types sometimes surprise families. 
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