Mixing blood types

As I understand them, the rules for blood transfusion don’t make sense.

People with type O blood can donate blood to anyone, including those of type AB. So it would appear that O and AB blood can mix without problems.

But type AB people can apparently only donate to other AB’s. Why can’t type O people receive type AB blood?

The A and B refer to specific antigens, which are things that can create antibody reactions. “O” just means that neither the A nor the B antigens are present.

These antigens are on the outside of the red blood cells. Give A, B, or O blood to someone with an AB blood type and their blood doesn’t react - they “detect” it as normal because it matches their own blood cells.

An O person who gets A, B, or AB blood has their immune system react to a “foreign” invader and the body goes berserk.

A and B are (simplistically) types of antigens, or things against which you might have an antibody. If you have an antibody against either A or B, and you get blood with that antigen, your blood will attack the red blood cells being transfused, which would be Bad.

So, type O people have anti-A and anti-B antibodies, so if they get either A, B or AB blood, it will be attacked.
Type AB people have no antibodies, so they can accept anything,
Type O people have no antigens, so nothing will attack it (there is no “anti-O antibody”)

“A” and “B” are the names of proteins on the red blood cells that can cause immune system reactions. Type O blood doesn’t have either A or B protein, which means it can be given to anyone without causing an immune system reaction. Type AB blood, on the other hand, has both proteins on it, which means that if it’s given to anyone whose immune system reacts to either the A or the B protein, it’ll cause an immune system reaction. Does that make sense?

In other words, if you were to give AB blood to somebody with the A blood type, their body would react to the B protein, and they’d get sick. If you gave it to somebody with the B blood type, their body would react to the A protein, and they’d get sick. If you gave it to somebody with the O blood type, their body would react to both the A protein and the B protein.

Thanks, that explains it nicely.

Genetically, A & B are co-dominant. So AB is always a genetic AB, type O will always be a genetic oo. A can be AA or Ao, b can be BB or Bo.

So an AB cannot make an oo baby, and a oo cannot make an AB. Anything else is genetically possible.

Well, no. You can be AA, or BB.

Yes, if you know you are AA or BB, you know you can’t be the parent of an O child. But if all you know is you are type A or B, you can’t rule out being the parent of an O child.

TV shows routinely ignore this fact. “The blood type doesn’t match mine or the mother’s. I can’t be the father.”

Frankly, it’s not that cut and dried. A person with O-type blood may have a single protein synthesis defect that prevents him from making the B antigen. If that person marries someone with AB blood, their offspring may inherit not only the A from the AB parent, but could also inherit the corrected gene for making the B antigen. Voila, AB offspring from AB and O parents.

Many other such subtle variants are possible.

Besides, what does this have to do with the OP?


Slight hijack, how much “foreign” blood does it take for the body to “go berserk”

Slight hijack #2: What is the rarest type of blood?

Hijack #3: What effect do positive and negative Rh factors have in all this?

Rh + and - are separate. If you’re Rh+, you don’t make antibodies against the Rh factor, so you can accept Rh+ or - blood. If you’re Rh-, you’ll see the Rh factor as foreign and attack it, so you can only accept Rh- blood.

Bombay? I’m not 100% sure if it’s **the **rarest, but certainly one of the rarest.

AB is the rarest worldwide. The most common group varies from region to region and ethnicity to ethnicity. In the USA O is most common.

As I understand it, though, an RH negative recipient only reacts to the second contact with RH positive blood.

Good call. Ignorance fought - mine, that is.

Slight nitpick, you only make antibodies against Rh+ after the first exposure. So that means someone who is Rh- can get Rh+ once. If they receive it a second time they will have a immune response, but not the first time.

ETA: Damn too slow! Exactly what Chronos said.

Another nitpick: an Rh negative person is only about 50% likely to make anti-D (that’s the main Rh antigen, the one that determines if you are Rh + or -) after one exposure to Rh + blood.