Most people know that there are various blood “types,” as defined by two major contributors: protein antigens on the surface of erythrocytes (red blood cells, or RBC’s) classified as either “A” or “B”, and a plus or minus designation rooted in the Rh factor (“Rh” derived from the Rhesus monkey). Other immunological factors exist, but do not normally affect compatibility in the realm of blood transfusions.
As a result, there are eight blood types: A, B, AB, and O, each with its own + or – appendix.
Out of the gate, it is important to recognize that people build antibodies against antigens that do not exist within their own systems, and do not mount such defenses against their own antigens (well, they sometimes do, and this is what results in autoimmune diseases, which can be severe or even fatal, as with lupus). The implications of this is that if you have blood type A, you will have anti-B antibodies in your blood and conversely. If you are type O, meaning you have neither A nor B antigens on your RBC’s, you have both anti-A and anti-B antibodies floating around.
Things are the same with the Rh-factor. If you are Rh-negative, your bodies produce anti-Rh antibodies. These can cross the placenta, meaning that if an Rh+ male mates with an Rh- female and the fetus turns out to be Rh-, the fetus–if Rh+, the genetically likely outcome–can be attacked by the potential mother’s circulating anti-Rh antibodies. This is the reason underlying the early-term administration of RhoGAM, which consists of anti-Rh antibodies that work in a seemingly paradoxical fashion: given to Rh- pregnant women, they desensitize the woman’s own response to the presence of an Rh+ fetus and prevent hemolytic disease of the newborn, which would otherwise kill at least 10,000 brand-new babies every year in the U.S. alone.
So, breaking things down, it’s easy to see why certain blood types do not mix. My own very common blood type is O+, which means that I could donate to anyone who is type A, B, or AB (since there is nothing for their anti-A or anti-B antibodies to attack) as long as the recipient is also Rh+. This is why people with blood type O- are called “universal donors.” Similarly, someone who is AB+ makes no antibodies to anything relevant, and is thus a “universal recipient.”
These factors are nicely summed up in this graphic:
You might rightfully wonder why someone with, for example, type-O blood–whose serum contains type-A and type-B antibodies–can be given harmlessly to those with various other blood types, since the donor blood could theoretically antagonize the RBC’s of the recipient. For whatever reason donor antibody-recipient antigent reactions are negligible compared to the parallel reverse situation. More importantly, when people receive blood, it is usually in the form of “packed RBC’s,” meaning that the serum of the donor–and along with it, all antibodies–has been stripped out and that only RBC’s themselves are given. Nevertheless, for other hematological reasons, the possibility of graft-versus-host disease exists and remains an important consideration in the transfusion milieu.
I wrote this post almost entirely off the top of my head, so it may not be entirely trustworthy. If I glitched anything, I trust that someone will correct me. But the bottom line is that your blood type not only needs to be a part of your medical record, but is also something you should know yourself. In my experience, most people have no idea. So don’t get into a car crash or other evil, unpredictable situation if you are among the unknowing.