Your body's defenses don't work properly against foreign blood, and that's the best scenario for a botched transfusion. You knew that blood types have to be compatible, but did you know that you also have to check for allergies? "I once had a lady that had a bad allergic reaction after receiving plasma. It turns out she had a peanut allergy and the donor had ingested a PBJ prior to donating, which caused the patient to go into anaphylactic shock." Note: If that sounds like bullshit, it's not.

That's one of many reasons finding the right blood to transfuse is much more difficult than ensuring that you've selected the right type -- although that's tricky too, because there are far more types than you were taught, and some are all but impossible to find. "There are over 30 other blood groups other than your standard A, B, O, AB. There is even a blood type called 'Bombay' which naturally makes an antibody for every other blood type. People with this type cannot get blood from anyone except from others with Bombay. It's extremely rare -- about four in a million."

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Giving your magic blood an ounce-for-ounce value somewhere between liquid gold and printer ink.

Yeah, if you lost that genetic lottery, don't expect the hospital to have any spare blood for you when you accidentally run your arm through a wheat thresher. You would think this would mean they would take any and all donors to cast as wide a net as possible, but ...

Considering the importance of transfusions, every donor should ideally be greeted with a hug from their favorite celebrity when they decide to show up and literally bleed for a complete stranger. But for decades, male donors have instead been greeted with intrusive questions like "Have you ever had another man's dick inside you?" and were shown the door if the answer was yes.

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"Door is on your left. Would you prefer to leave to the sound of whispered accusations, or quiet glaring?"

"Since 1977, the FDA has not allowed men who have had sex with other men, even once, to donate blood," Dr. Matthew told us. "This was the result of the AIDS epidemic and the lack of knowledge and testing limitations. Well, now we've got a really good handle on HIV testing. Your chances of contracting HIV through transfusion is like one in two million." In others words, you have better odds of getting struck by lightning, and then discovering that the scar formed the shape of winning lottery numbers.

The ability to reliably test for HIV came about in 1985, and all blood donors are screened for it, so the FDA somewhat relaxed the rule prohibiting men who've had sex with men from donating ... in December 2015. "If [gay and bisexual men] haven't had sexual contact with a person at risk for HIV within the past year, they can donate."

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"FINE! But this counts as your social progression for next Christmas, too."

Yep, monogamous, sexually active gay and bisexual men who use protection still can't donate, while heterosexuals who have nonstop nasty, unprotected sex can. This is despite the fact that modern tests only need a waiting period of 25 days after sex to detect signs of HIV, although that would still rule out any same-sex couples who weren't going through a bit of a rough patch. But to be fair, maybe the rules wouldn't be so strict if not for the fact that ...

Dr. Matthew also encountered a bizarre example of peer pressure. "I had a high school donor fake parental consent so she could donate with friends. She had a seizure during the donation and had to be hospitalized. She had a disorder that she didn't reveal during screening, but she donated because she wanted to be with her friends." Remember, kids, if your friends only like you when you're donating blood, they're not really your friends.

Our other source, Ann, also deals with this kind of nonsense. "One of my responsibilities is to process recalls on blood. The majority of them are from donors who, at their last donation, suddenly 'remembered' they had previously engaged in behavior or had a disease that would preclude them from donating. Then all products they've donated need to be recalled."

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"I've never thought this was a big deal before, but to be safe, how big an issue is leprosy for you guys?"

This is a problem, because as you've heard in thousands of PSAs, hospitals aren't exactly swimming in blood. In fact ...

"Once," says Dr. Matthew, "I stayed up all night freaked the hell out about anything coming through the door of the emergency departments at the hospitals we service, because we only had four units in the entire system." So Dr. Matthew ordered more blood from a blood bank -- the facilities that store extra for emergencies -- only for his order to be held up at the airport and miss its flight. "If a trauma had shown up or a surgery turned into a massive transfusion situation, any or all of those patients would have died and there'd have been nothing we could do about it."

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"Wait, does anyone know how to get our elevators to do that thing from The Shining?"

That problem is compounded by the fact that hospitals do more transfusions than they need to, as Ann explains. "Some of it might be due to ignorance of what blood [is] expected to do, and what sort of blood levels a patient can handle before transfusion is necessary. We see doctors 'topping up' their patients before a holiday weekend." They're like goddamn reverse Draculas.

It's not just a matter of waste, either -- overtransfusion can result in elevated blood pressure, reduced blood to the heart, and that infamously troublesome side-effect of death. Yet hospitals keep excessively topping off your vein oil, thanks to misinformation on when a patient needs blood. And on the other extreme ...