Here is a good question from one of our readers: “Why is blood group O+ not a universal donor?”

It’s pretty easy to determine which blood types can donate to which other blood types, if you’re working in the field or if you’ve just had a (good) lecture on it. But if you’ve been away from transfusion medicine for a while, it can seem like Greek when you’re faced with it again.

The most important transfusion rule
The main point to remember when thinking about the appropriate blood type to transfuse is that the thing you’re most worried about is antigens on the surface of the donor red cells. The biggest danger is that you’ll put donor red cells into a patient who has antibodies against those donor red cells…and then the donor red cells will get busted open and bad things can happen.

We don’t worry too much about the opposite situation (donor antibodies attacking host red cells) for a number of reasons. One reason is that there are A and B antigens on cells other than red cells. Weird, right? So any donor anti-A or anti-B antibodies you infuse into the patient have lots of places to bind, which lessens the amount of binding to red cells. Another reason has to do with volume: the relatively small amount of antibodies in a unit of donor red blood cells is quickly diluted in the patient’s (much bigger) blood volume, minimizing the effect the antibodies would have on the patient’s red cells.

Another thing to remember
One other point to remember is that antibodies in the ABO system are present from birth (you don’t need to be exposed to type B red cells to make type B antibodies). You just make antibodies against whatever antigens you don’t have on your own red cells. I know, it’s weird, and it goes against what is taught about antibody formation. Just accept it for now! Maybe we can talk about it in another post.

Antibodies in the Rh system, however, are acquired, meaning that they develop in the typical way (after exposure to the antigen). This means that if you are Rh negative, you will not make anti-Rh antibodies unless you are exposed to Rh positive blood (usually through a transfusion or a pregnancy with an Rh positive fetus). Rh positive people, of course, won’t form anti-Rh antibodies.

Here’s how to find a safe blood type to transfuse
So how do we use this information to tell what blood types can be transfused into other blood types?

For the ABO system, look at the host blood type (A, B, AB or O). By deduction, figure out which ABO antibodies the host has. As you can see in the table above, the host will automatically have antibodies against whatever antigens are NOT present on the host red cells. You can’t give the host any donor blood that has antigens for those host antibodies to bind to. Put another way, you can only give that host donor blood that does NOT have red cell antigens corresponding to the host’s antibodies. That’s it!

For example, say a host has type A blood. That means the host has anti-B antibodies (automatically!). Which means that you can’t transfuse any red cells that have B antigens on their surface (this includes types B and AB). So type A or O donor blood would be fine for the type A host. Another example: if the host has type O blood, then the host automatically has both anti-A and anti-B antibodies, and you can’t transfuse any red cells that have A or B antigens (this includes types A, B, and AB). The only donor type that you can transfuse into a type O host is type O!

Universal donor
This brings us to the “universal donor” concept. Type O blood has neither A nor B antigens on the red cell surface. Which means that there is nothing for anti-A or anti-B antibodies to bind to…which means that you can safely put type O blood into any patient (A, B, AB, or O). There are no A or B antigens on type O blood, so there is nothing for host antibodies to bind to! That’s why type O is considered to be the universal donor. You can remember this by remembering that donor has an “O” in it (donOr).

Antibodies in the Rh system, as we mentioned, are formed only after being exposed to Rh positive blood. So a patient who is Rh negative might have anti-Rh antibodies (if they’ve been exposed to Rh positive blood) or might not (if they’ve never been exposed to Rh positive blood). So the universal donor as far as the Rh system is concerned is Rh negative blood. Since an Rh negative host may have anti-Rh antibodies, you wouldn’t want to put Rh positive blood into an Rh negative patient.

Which brings me to one last point. If it’s an emergency, and you only have Rh positive blood, you can sometimes transfuse that Rh positive blood into an Rh negative host. You have to be sure the host does not have anti-Rh antibodies, of course, and if that is the case, you can safely transfuse Rh positive blood. You can only do this once, however – because after being exposed to this Rh positive donor blood, the host will now make anti-Rh antibodies (and the next time the host needs a transfusion, you won’t be able to use Rh positive blood). So if possible, you should always use Rh negative blood for Rh negative patients.

Bottom line
The universal donor for both ABO and Rh systems is type O negative. You wouldn’t consider O positive blood to be a universal donor, because you can only give Rh positive blood to patients who do not have Rh antibodies.