You’re squared off against your foe. You’re usually a good shot, but the rain is coming down pretty heavily and besides, you need to do some long division in your head if you’re going to get that bomb defused in time. Not to mention you’re bleeding pretty bad from the gunshot wound your foe gave you moments before.
A shot rings out, and you both fall.
Luckily, you manage to drag yourself to the bomb and type in the code before you pass out from blood loss. You wake up in the hospital and the doc is about to give you a transfusion. You thank your lucky stars for Karl Landsteiner, because without him there was a good chance you wouldn’t survive the procedure.
Landsteiner is the man who figured out blood types, and won himself a Nobel prize for it.
You have antibodies which attack intruders in your body. So if a germ turns up and you have the right antibody, it will take care of that for you, in the mafia sense of the word. Things which trigger antibodies are called ‘antigens’ (ie, antibody generators).
Now blood cells aren’t antigens; they don’t trigger any antibodies, because that would be a terrible idea. But, and here’s the wrinkle when it comes to blood transfusions, you can have antigens covering your blood cells.
Now this obviously isn’t a problem for your own personal blood. You see, there are two antigens that can cover your blood cells – let’s call them type ‘A’ antigens, and type ‘B’ antigens. If your body produces blood cells covered in ‘A’ antigens, then your body will recognise those antigens as safe, and be perfectly happy with them. No trouble. So if you lose some blood, and you get more blood from someone else whose blood is also type A, then your body still has no problem with this. It sees blood, the antigens check out, still no problem.
But if you get blood of type ‘B’, well then you have a problem. Your antibodies see this ‘B’ antigen covering the cells and don’t recognise it. They start to attack and attach themselves to the blood cells, which start to clump together, and now you have clumps of blood whizzing around your body and you’re in trouble.
This will obviously happen the exact same way if you’re type B, and you get type A blood.
So A can give to A, and B can give to B.
But people can also produce blood with both A and B antigens around the blood cells, so you get ‘AB’ blood – AB people can receive either A or B blood (as their bodies will recognise both A and B antigens as friendly), but they can’t give their blood to someone with just A or just B type blood (as the recipient’s body won’t recognise the antigen it doesn’t have).
You can also have people who don’t have any antigens surrounding their blood cells – we call this blood type ‘O’. So O type blood can be given to anyone, as it doesn’t have any antigens which might bother any antibodies.
The other major factor in blood types you’ll hear about is called the Rhesus group; you’re either Rhesus-positive, or Rhesus negative. It works on the same general principle as the ABO system, so if you’re Rhesus-negative then you can give to anyone, but Rhesus-positive can only give to other Rhesus-positive people.
Which is what people mean when they say you have A positive, or AB negative, or whatever, blood types.
Naturally there are other factors at play, and you can read this article for a little more detail, and then follow links as far down the rabbit hole as you’d like to go…