What is the science behind naming of blood groups? Like what is that blood contains which makes it O-positive or O-negative?
In: 10
Your blood type is the kinds of molecules on the outside of your red blood cells, called antigens. There’s a lot of variation here, often thought to be a way to make it harder for parasites and bacteria that infect red blood cells to infect us.
The A, B, and O are the main molecules most relevant when it comes to blood transfusions, but there’s actually a *lot* more. A and B are the names of two types of red blood cell surface molecule, and O is for if you don’t have either an A or a B. So for example, someone with type A blood has the A antigen but no B antigen. Someone with AB blood has both A and B antigens on their red blood cells. Someone with O has neither. [Here’s a chart from Wikipedia showing it.](https://en.wikipedia.org/wiki/ABO_blood_group_system#/media/File:ABO_blood_type.svg)
If you have heard of people’s blood types described as positive or negative, such as A positive or O negative, this refers to the Rh blood group. It’s called that because it was first discovered by studying Rhesus monkeys, but is abbreviated with the letter D. If you have it, you are D positive. If you don’t have it, you are D negative. There’s many rarer forms where people have partial D antigens and stuff, so this also gets quite complicated.
In reality, our red blood cells have a *lot* more surface molecules, which are all given letter designations, but for blood banking, as long as you match the A, B, O, and Rh type, for the majority of people there will be no problems, so this is as far as most people’s blood type gets determined.
The naming corresponds to blood cell surface proteins to which the human immune system reacts. The immune system is on constant surveillance for living cells that appear foreign, since we have evolved to kill things that can kill us, like bacteria. Our immune system primarily reacts to protein structures on the surface of cells.
This is really only consequential for blood transfusions. If you are type A, you have A type proteins on your blood cells, and your immune system is cool with blood from donors that also has type A surface proteins, and also cool with type O blood, which corresponds to “neither A nor B.” infusion of blood cells with B surface proteins will cause a massive immune response and kill you.
If your blood type is B, B is ok and O is ok, but A will kill you. AB means you have both A and B proteins, and can therefore accept any type of blood and only donate to AB people, since those with any other blood type will react to AB blood. O can donate to anyone, but only accept O.
Positive and negative corresponds to another whole set of surface proteins which can potentially cause immune responses by mothers to the fetus they are carrying, if mom is a negative blood type and is carrying a baby with a positive blood type.
So red blood cells have proteins on them. the O type have none which is why they are universal givers, A type has one set of proteins and B type another. Essentially A is triangles attached to the cell and B squares. The protein type not in your body your body will attack so a O person will have antibodies against both triangles (A) and Squares (B)
Then there is the rhesus factor which you either have or not. If you are rh+ you can recieve any blood if you are rh- you have to have rh- blood because again you have antibodies against rh+.
https://www.blood.co.uk/news-and-campaigns/the-donor-magazine-summer-2017/blood-group-basics/
More info can be found here
In humans, there are three major antigen groups on blood cells: A, B, and Rh(D). Each of these are made up of one or more proteins on the surface of the blood cells that may trigger an immune response.
There are quite a few other antigen groups as well. These others tend to be either much rarer they are either much rarer or clinically irrelevant. One of the other important ones is DI, which is only found in Asians and Native Americans. Many are clinically irrelevant as the donated blood will naturally convert to match the recipient’s blood.
By convention, blood that lacks both A and B is called O, otherwise which of the two groups exist on the cells is listed. So, O blood lacks both A and B; AB blood has both A and B; A blood has only A; and B blood only has B. The positive/negative refers to the presence or absence of the Rh(D) protein.
So, with all of this, O- blood lacks the A, B, and Rh(D) proteins. O+ blood lacks A and B proteins, but has the Rh(D) protein.
Additionally to the proteins, blood that lacks the protein will also have antibodies against the protein. So, somebody with O blood is allergic to both the A and B proteins.
There are 2 things at play.
The ABO type, and the rhesus factor.
Your blood can have A or B or both or neither of the AB-antigens. If you have A, you are A, if B then B, if both, you have AB. If you have neither, you have O, think of it as *zero* rather than a letter. Zero AB-antigens = 0.
The rhesus factor is a different molecule, and it is just either you have it (+) or don’t (-).
And from these you get the A or B or AB or 0, and the + or -.
As for what these antigens do, is a bit beyond the scope of an ELI5 question. Their purpose, in extremely simplified terms, is to boost the immunity of our blood against certain infections or parasites etc.
I’ll add a little bit too
When they discovered the A and B antigen, they expected to find more related antigens, so started by calling the first 2 A and B with the plan to continue with the alphabet. However they then realised that there were only those two and that they existed in isolation, combined or not at all. A and B had become established as names already and it also worked nicely as a simple ways of describing the combination someone had so they stuck with it.
It was later they started to find other antigens, but they weren’t variants of the A and B so they got called other things like rhesus.
There are more antigens than A, B and rhesus, it’s just that in blood typing for transfusion they usually don’t matter than much. It’s only if there is a suspected problem or requirement that they are checked in clinical practice.
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