ELI5- What are blood groups? Why do they differ from person to person? Why can’t a person of one blood group receive blood from a person of a different blood group?


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In: Biology

There are four groups of blood types, A, B, AB, and O. They can all be positive, or negative.

what makes you fall in one of the groups is whether or not you have A and/or B antigens, or none of them:

> A type blood has only A antigens on red blood cells
> B type blood has only B antigens on red blood cells
> AB has both A and B antigens on red blood cells
> O has neither A nor B antigens on red blood cells

As for the plus or minus, that depends on if you have a protein called RhD on your red blood cells (positive meaning you have it, negative meaning you don’t).

The way blood can be transferred works like this: You can only receive blood from blood types that don’t have any antigens you don’t have. So, O can give to any type, A and B can give to AB and their own types, and AB can give to only themselves.

On top of that though, while negative blood types can give to both positive and negative blood, positive blood cannot transfer to negative. So, while O- can give to literally any blood type, O+ can only give to positive ones.

They’re basically genetic markers that you have in your blood.

If the donor has a marker that the recipient doesn’t have the body will detect it as foreign and the immune system will attack. Which can cause blood clotting, kidney failure, pain all over, breathing problems (and possibly death)…all the bad stuff that can happen when bloodcells are breaking down inside your body.

Even if you have the same bloodtype you might still get an allergice reaction, which is why doctors always perform a compatibility test before doing a transfusion.

As for why they’re different? Some people are tall, some people have red hair, some people have brown eyes, some people have AB+ bloodgroups. There is really no reason except that at some point someone got the genetics for these markers and then their kids got them etc.

Your blood cells have little markers on them that say ‘I belong to this body’

If you give someone blood with the wrong tag, then your immune system starts attacking the other person’s blood thinking it’s an invader causing an allergic reaction. Which is bad.

In some blood types, it’s like the blood cells are wearing little hats. If you get some blood cells wearing hats and you don’t normally have them, your body is like, whoah, these shouldn’t be here, and it attacks them. Type A only likes type A hats, and type B only likes type B hats. Type AB is cool with either kind of hat. Type O doesn’t have any hats itself, so the bodies of Type O people will attack A or B or AB blood cells. People with A, B, or AB blood types are ok receiving the hatless Type O blood cells.

The positive and negative refers to Rh blood types. Rh blood types are like the cells have mustaches. Rh+ types, they’ve all got mustaches but they’re ok accepting blood cells with no mustaches. Rh- types, they don’t want any of those stupid mustaches. Maybe they could tolerate it a bit, if it’s an emergency, but really no mustaches would be better. So an AB+ person is cool with blood that has any kind of hat and mustaches or no mustaches, so they can take any kind of blood, while an O- person can only accept blood with no hats and no mustaches.

There are other accessories that affect blood type, representing more detailed blood types. For surgery and stuff usually ABO type and Rh type are all that matters, but for some transplants you really need the closest type possible, like blood with the exact same hat and facial hair and jewelry and clothes and haircut. That’s why people who need kidneys or bone marrow will sometimes ask lots and lots of people to check if they’re a potential match, and maybe never find anyone.

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