Blood Compatibility: What it Means, Why it Matters

Every two seconds, someone in the U.S needs blood. And not just any blood: the donor and the patient must have compatible blood types.

Otherwise, the transfusion might do much more harm than good. But what does that actually mean for patients and donors?

What Are Blood Types, Anyway?

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You’ve probably heard of the eight blood types: A positive, A negative, B positive, B negative, AB positive, AB negative, O positive, and O negative.

These types are differentiated by the presence or absence of certain proteins on the red cell surface: A, B and Rh. Someone with A positive blood, for example, has the A antigen and the Rh antigen, but not the B antigen. Someone with AB positive blood has all three antigens, while O negative blood doesn’t have any of them.

Some Blood Types Just Don’t Get Along With Each Other

Each of those eight types is “compatible” with a few other types. This means they can be given to patients interchangeably. If you ever need a blood transfusion, you’ll be given blood from someone with your exact type or another, compatible type. Compatibility ensures that, even if you have a rare blood type, doctors can always find blood to treat you.

O negative blood is universally compatible: it can be given to anyone. This is crucial when the patient’s blood type is unknown and there’s no time to find out – such as when a trauma patient arrives at an emergency room. In that case, doctors rely on O negative blood. (If the patient needs plasma, they will be given plasma from an AB negative donor, which is also universally compatible.)

Learn which blood types are compatible – download a free blood compatibility chart here.

Where Do Blood Types Come From?

The type of blood you have was passed down from your parents, just like your eye and hair color. Your siblings and other close relatives may have the same type as well. And it’s not just your family – you’re likely to have the same blood type as other people in your racial, ethnic and geographic groups.

For example, type O blood is the most common type all over the world, but while close to 100% of South Americans have it, less than 60% of people in central Asia do. On the other hand, type AB blood is rare everywhere and only 4% of Americans have it – but nearly 10% of people in Japan are type AB.(verification source of this information?)

Diversity Among Blood Donors Ensures Compatible Blood for Everyone

Blood compatibility within ethnic groups goes deeper than a likelihood of being type O vs type AB. Those 8 types are just the highest level, most common ways of grouping the differences in our blood. There are many other antigens that we can have on our blood red blood cells, and they can influence how successful a transfusion is – the best transfusion results generally come when the donor and recipient are of similar race and ethnicity.

For example, some sickle cell disease patients require monthly blood transfusions to lessen the pain of their disease. Since more than 90% of Americans who have sickle cell disease are of African descent, blood from African-Americans donors is most compatible for them.

CMV Negative Donors Can Help Save Babies

A different type of compatibility involves antibodies that donors and patients may have in their blood. Antibodies for CMV (Cytomegalovirus) are very common – up to 85 percent of us will have had this flu-like virus by age 40. As with other viruses, once you’ve had CMV, your body will retain the antibodies needed to fight the infection. Thus, if you’ve had CMV you are “positive” for the antibodies; if you’ve never had CMV you are considered “CMV negative.”

CMV is generally harmless to healthy kids and adults, but it can be fatal to babies and cause serious problems for people who have weakened immune systems. For this reason, babies and people who are immunocompromised should only receive CMV negative blood. (Of course, it must also be a compatible type – and with only 15% of the population being CMV-negative, that can be challenging.)

Your Blood Type Is Needed – No Matter What It Is!

You’ve probably guessed that donations of type O negative blood (the “universal” blood type) and AB negative plasma are always needed. Donations from African-Americans who do not have sickle cell disease (having sickle cell trait is fine) are in demand as well. Lastly,donations from people who are CMV negative are critical for certain types of patients.

But it’s also true that donations of every type of blood are needed. Perhaps you have O positive blood – so common, and therefore so much in demand. Or maybe you have a rare type, meaning donors are scarce. Or maybe you don’t know what type of blood you have.

If you are willing and eligible to donate blood, your donation can help save lives. Unfortunately, fewer than 10% of the U.S.population actually donates blood. So whether you are O-positive or AB-negative or anything in between, your blood is needed!

Be a hero – schedule a visit to a Red Cross blood drive today.




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