Not everyone's blood is exactly the same. Blood from one person may have small differences when compared to blood from another person. We use these differences to group blood into categories we call "types." There are eight common blood types.
So, how do we know one blood type from another? We use two characteristics. First, we look for A and B antigens. These are molecules on the surface of red blood cells. Based on the presence or absence of these antigens, we divide blood into four basic groups. We call them "A, B, AB and O."
Next, we look for a protein called the "Rh factor." If it's present, we say your blood is "positive." If it's not, we say your blood is "negative." By combining what we know about your A and B antigens and your Rh factor, we get the eight common blood types.
Why do we care about blood types? Well, not all are compatible with each other. That matters when we want to take blood from a donor and give it to someone else, a process we call "transfusion." So before a transfusion, we always make sure the blood types are compatible.
To find out your blood type, you can have a sample of your blood tested by a lab. To learn more about what your blood type means for you as a blood donor or recipient, talk to your doctor.