So if I receive an organ transplant, I need to be put on immune suppressive drugs to keep my body from rejecting the transplant. If I go into surgery for some other reason and receive several units of blood, why doesn’t my body reject all that foreign blood coursing through my veins?
It may be because the vast majority of blood cells, including the red cells (by far the most numerous), do not have MHC (HLA) proteins.
The surface glycoproteins that red blood cells do express are the ABO proteins, are you are type-matched for that before a blood transfusion.
Blood cells are quite simple, compared to most body organs.
So we can classify blood into a few major types, and match people to the proper blood type before transfusions. So then the blood is close enough to your own that there are no serious reactions to it.
Sort of like buying a new appliance for your kitchen, and matching the color to your current appliances. Fairly easy to do, because there are only a few colors used for appliances. But matching drapes, carpet & furniture in a living room is much tougher, because there are so many more colors & patterns in both of them.
Is the reaction from mismatched blood an immune response, then?
Yes, it is an immune response. The body recognizes that the group in the cell surface is not the right one, and it will attack and destroy those red blood cells.
Blood is also replenished by the body at a steady rate. The kidney you have transplanted into you will be the same kidney in a year. The blood you were transfused with will have been replaced completely with your own blood several times over in that same time.
Actually blood cells express far more antigens than just the ABO and Rh glycoproteins. There are 250+ known antigens that are expressed on blood cells, although only about 12 are clinically important enough to deal with on a day to day basis in a blood bank.
You need two things to have a reaction to a transfusion. The transfused blood cells need to have an antigen on their cell surface, and you need to already have antibodies against those antigens. The more you have of either, the stronger the reaction.
The ABO antigen group is super important because it is not unique to humans. Animals, microbes, and possibly plants all express it and if you don’t, your environmental exposure to those antigens will prompt you to develop antibodies against them within the first 6 months of life. If you are given a transfusion of AB red blood cells, and you are type O, you are in for a world of hurt. You provide the anti-A and anti-B antibodies, and the transfusion provides the A and B antigens.
Other antigens, such as Rh factor, are less important because you don’t naturally generate antibodies against them since you don’t typically encounter them in your environment. You DO, however, generate antibodies against them if you are exposed to blood with that antigen. So for example, if you are O- and have never been exposed to the Rh antigen, and you get an O+ transfusion, you will probably have a non fatal allergic reaction. If you get a second O+ transfusion a week later, it will probably kill you. For this reason, they often give a drug called RhoGAM which binds to the Rh factor and prevents you from reacting or becoming sensitized to it.
This can lead to problems when you have an Rh - mother and an Rh + fetus. For that reason blood bank docs like knowing about pregnancy history because having a bunch of kids can increase the risk of having a transfusion reaction.
Like I said before, there are many antigens that aren’t even tested for. As a result, allergic reactions to blood transfusions are fairly common, so you should be ready for hives, and a fever. As long as the blood you receive is properly matched to your ABO antigen group, you should be okay. Keep in mind though, that there is a lot of stuff going on when you receive a transfusion, and even if everyone does their job right, things can still go terribly awry.
And what happens exactly if, being O, I get AB blood? Doesn’t it take some time for antibodies to multiply and begin to seriously attack the transfused blood cells? Or will it kill me in short order? And if so, how?
Let’s assume for instance that I need immediatly a transfusion (or I will die in quick order) and for some reason there’s no compatible blood available? Would I be better off receiving AB blood while I’m waiting for proper medical care (say, some hours or one day later) or would it be pointless?
It was explained by heavyarms553: those two antigens, A and B, are also present in sources other than human blood which anybody will encounter. The antigens are developed when the antigen is first encountered, whether that happens due to a blood transfusion or because a bactery with it happens to enter the bloodstream. So by the time that an O patient gets a first AB transfussion, they already have the antigens and the reaction will be immediate. Rather than have its resources augmented by the incoming blood, the patient’s body starts spending them fighting it.
The gold standard of treatment in emergency care is to transfuse O- blood, which is the “universal donor.” It is guaranteed not to react with the patient’s ABO group antigens. You would NEVER give AB blood to someone who hasn’t been typed as the odd of killing them are higher than not. The problem is that everyone wants O- blood, so blood banks often have little to spare. That can be problematic when the blood bank runs out, and health care workers are forced to make some difficult decisions. In fact, blood bank triage is something taught at medical schools.
I noticed that, but it doesn’t mean that there’s a lot of antibodies present. If I’m not mistaken, when you’re infected a second time by something, some corresponding antibodies are present but they need to be “produced” in large quantities to fight off the infection, which, I assume, takes some time.
Besides, it doesn’t answer my question about what will happen to me exactly. If it’s lethal, how will I die, for instance? How much time will it take?
Actually, the gold standard is exact matching. O- is just used so often because it’s so rare that you can get an exact match in an emergency situation.
A person with an O blood type will have a large number of anti-A and anti-B antibodies already in his blood. Remember, you are constantly being exposed to, and subsequently innoculated against, A and B antigens. That is why you have such a high concentration of those antibodies. Since the antibodies are already circulating, an ABO blood group mismatch reaction is fast and severe.
As for how a transfusion reaction actually kills you, it depends. The more blood you receive, the stronger the reaction. The typical symptoms of an acute hemolytic transfusion reaction are fever, chills, rigors (violent shaking that you see sometimes with the flu or malaria), back pain, kidney failure, disseminated intravascular coagulation, hemorrhaging (from the DIC), and clot problems (strokes, heart attacks, osteonecrosis, etc).
All of those problems are basically the result of the transfused blood turning into sludge as it gets attacked by your immune system. In fact, that is how blood is typed in the laboratory. They mix blood with pure anti-A, anti-B, and anti-D (Rh factor) antibodies and if the mixture turns into a semisolid sludge, then you know that that blood contains that antigen.
Many years ago, in junior high science class, we actually got blood from several volunteers. They poked their finger, and squeezed a drop of blood out onto a paper. When 2 drops of non-compatible blood were combined, we could actually see (with our eyes, no microscope needed) the 2 little drops of blood reacting and clumping into little clots on the paper. Imagine what that would do with a whole pint actually inside your veins!
O- blood is the gold standard in emergency care because you don’t have time to type the patient. Yes, it would be ideal to Type, Screen, and Cross every patient who comes in, and that is exactly what you do if you know your going to need the blood ahead of time, but if you have someone who is going to bleed out in 5 minutes if you don’t do something, you just give O- blood.
You can always use ‘coconut-water’, too, as per Cecil 
But this is not really along the OP, of course. There are also blood ‘substitutes’ now, but iirc are not approved in USA (yet?).