What are blood types?
A blood type is a classification system that allows healthcare providers to determine whether your blood is compatible or incompatible with someone else’s blood. There are four main blood types: A, B, AB and O. Blood bank specialists determine your blood type based on whether you have antigen A or B on your red blood cells. They also look for a protein called the Rh factor. They classify your blood type as positive (+) if you have this protein and negative (-) if you don’t.
This makes for eight common blood types:
A positive (A+).
A negative (A-).
B positive (B+).
B negative (B-).
AB positive (AB+).
AB negative (AB-).
O positive (O+).
O negative (O-).
Knowing about blood types allows healthcare providers to safely transfuse donated blood from one person into another during a blood transfusion. Blood types also need to be compatible for organ transplants.
What blood types mean
Most people think of A, B, AB and O when they hear the phrase “blood types.” These letters classify blood types based on whether red blood cells have the A antigen or B antigen. This is called the ABO system.
Type A: Red blood cells have the A antigen.
Type B: Red blood cells have the B antigen.
Type AB: Red blood cells have both A and B antigens.
Type O: Red blood cells have neither A nor B antigens.
Blood types are either “positive” or “negative,” depending on the absence or presence of the Rh factor’s D antigen, another marker. This is called the Rh system. Being RhD positive is more common than being RhD negative.
Positive (+): Red blood cells have the RhD antigen.
Negative (-): Red blood cells don’t have the RhD antigen.
Universal donor
Blood type O negative (O-) is the universal donor. This means that a person with any other blood type can safely receive your blood. It doesn’t contain any antigen markers that other blood types recognize as not belonging. Providers use type O negative blood the most in emergencies when someone needs blood fast.
Providers look at different markers to determine blood type compatibility for donating plasma. Plasma is the liquid part of blood. The universal plasma donor is type AB.
Universal recipient
Blood type AB positive (AB+) is the universal recipient. You can safely receive blood from any other blood type. Your blood recognizes all potential antigens as safe, so your immune system doesn’t launch an attack.
Can your blood type change?
Usually, you’ll have the same blood type all of your life. In rare cases, however, blood types can change. The change usually relates to unique circumstances, such as having a bone marrow transplant or getting certain types of leukemia or infections. Not all of these changes in blood type are permanent.
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