*Here’s another few questions from a neophyte: Why does everyone seem to prefer to send O Neg? Given the law of supply and demand, I can understand rationing it, but what also makes it so special for women and children? A lower chance of ‘body rejection’? *
All right, so here’s the simplest way I can think to explain it. There are hundreds (maybe more) of blood types. By this I mean the antigens on your red blood cell wall (we’ll keep HLA antigens and white cells out of this). But for the sake of blood typing and finding compatible blood we only do a routine ‘type’ for the ABO and Rh blood groups. Although we think of someone as A+ or O- or AB+, these are in fact two completely different blood antigens unrelated to each other (AB/O/A = ABO type, -/+ = Rh type).
Now your body will naturally make antibodies against the A and the B cell phenotype, i.e. if you’re a type O you will naturally have anti-A and anti-B, or if you’re a type B you will have only anti-A, and if you’re type AB you will have neither. This is what makes type O blood compatible with everyone. Now as far as the +/- though, the Rh group is the second most common group we type for. And although we designate someone as + or -, the Rh group consists of several antigens, the +/- is just what we know as the ‘D’ type. Rh actually consist of D,C,c,E,e, and many more antigens. The Rh phenotype is very complex, and fills up a ton of medical journals. The point that I was trying to make is that we just type for the D part of the Rh, and your body doesn’t naturally make antibodies against it. You must be stimulated into making antibodies against it just like you would against any other foreign antigen (like a vaccine) by being exposed to it. So if you’re O-, you are not likely to have any reaction the first time you are transfused with O+ blood.
Why we care if you’re a woman of childbearing age: Basically you NEVER want to give a woman Rh+ blood if you don’t know her type because of fears that she will develop an Anti-D antibody. This is a very clinically significant problem if she has a child with a man who is Rh positive and the child is then born Rh positive. This leads to hemolytic disease of the newborn (I love wiki).
Why we care if you’re a kid: basically, it’s purely sentimental… basically we just want to conserve as much O- as we can, but clearly if our inventory is enough we will tend to want to use the O- units. Being a child just makes us want to prevent the child from making an antibody more because, well, he’s just a kid. (I say he, because girls/women would fall under the category of never transfusing someone of childbearing age with Rh-positive units)
And just one more: I’ve seen a few war movies (and yeah, I know they’re not exactly medically accurate), with a few guys giving direct transfusions: donor connected to an IV to the receiver on his own IV, and a long tube between 'em. Does this ever happen? What’s the safety ramifications for doing it either in the hospital or on the battlefield?
I’m answering this with the equivalent of a bump, because I really don’t know. The closest example of this I can think of was a semi-recent incident in El Salvador that had our Blood Transfusion Service (BTS)(modern name of blood bank) buzzing.
To leave you with a final thought, although we, the BTS the doctors and the nurses, do all we can to make an emergency transfusion safe, there always carries a risk. There can be many types of reactions other than the ones involve having the blood being compatible. And even compatibility issues as well. A recent incident that comes to mind (although trauma ID’s are confidential, I am still using false ones) was when we had a young male gunshot victim come in early in the morning with an unknown identity. He was given a trauma name of TRAUMAMALE HDL, and the emergency department requested 4 units uncrossmatched. I can’t recall if we sent O- or O+, but it didn’t matter, once we got a sample and tested it as quickly as we could we identified an Anti-Kell antibody for another less common blood group. It turns out that this patient was later identified as a male who we had admitted under similar circumstances over 3 years ago for a stabbing wound. He developed the Anti-Kell antibody at that time making some of the units of blood incompatible. That patient did not live, though I don’t think it was directly related to the transfusion.