Donating plasma and donating blood are essentially the same process: the entry questionnaire, getting hooked up to a machine, the cookie afterward. But in the US there’s a key difference: one is an act of charity, and the other an act of commerce. So why is it that you get paid to donate plasma, but not blood?

It’s a common misconception that the Food and Drug Administration bans paying for blood. In fact, it only says blood from paid donors has to be labeled that way. But hospitals won’t use it. In practice, nobody really pays for blood, said Mario Macis, an economist at the Johns Hopkins Carey Business School who has studied incentives for blood donation. “Even though it’s legal, it’s still considered not totally moral or ethical to pay cash to blood donors.”

Aside from the ickiness of handing out literal blood money, the FDA worries that paying donors would jeopardize the safety of the blood supply. No one with a blood-borne illness is eligible to donate, but the agency worries that if money were on the line, donors might lie about their health or their risk behaviors.

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The science there is far from settled. But the World Health Organization finds it convincing enough that they discourage countries from paying blood donors. “Evidence shows significantly lower prevalence of transfusion-transmissible infections among voluntary nonremunerated donors than among other types of donors,” their commentary in 2013 read.

Donated blood is tested for diseases, anyway, but the FDA says it intends those steps to be redundant security measures, “like layers of an onion.”

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Plasma donation — in which blood is drawn, plasma separated out, and then blood cells and other components put back into you — is often compensated. The FDA doesn’t require paid plasma donations to be labeled. The reason is that plasma collected this way never goes straight into another person. It’s broken into many different protein products that will become pharmaceuticals. Along the way, these components are processed to remove or kill any virus stowaways. “The risk of infection is inherently much lower,” said Dr. Christopher Stowell, who recently chaired the FDA’s Blood Products Advisory Committee. Whole red blood cells are too fragile to undergo the same kind of processing as plasma.

And there’s some evidence that paying for plasma does, indeed, lead more people to conceal their disease status or risk behaviors. For instance, the Government Accountability Office looked at California’s blood versus plasma supply back in the 1990s and found that the plasma had much higher rates of HIV. There are reports of desperate donors lying about illnesses to donate for cash.

However, the type of compensation matters. In a 2013 Science paper, Macis and others found that rewards such as gift cards, coupons, and T-shirts almost always boosted donations, and they didn’t find any effects on blood safety. (The FDA doesn’t count rewards like this as payment, as long as they can’t be easily turned into cash.) “Nonmonetary incentives do work,” Macis said. He thinks using more of these motivators could help the United States manage seasonal blood shortages.

Were you hoping for more than a T-shirt? Don’t even think about selling a kidney. The National Organ Transplant Act of 1984 made it illegal to pay for organs. But in the 2011 case Flynn v. Holder, the US Court of Appeals for the Ninth Circuit ruled that a certain method of bone marrow donation could be compensated.

Traditionally, bone marrow was collected in a surgical procedure, with a hollow needle stuck straight into the pelvis. But in a more common method called peripheral blood stem cell apheresis, donors take drugs that release the stem cells from their marrow into their blood. Then they donate the cells through a needle in the arm and an apheresis machine — just like a plasma donation.

Centers that collect such cells pay up to $800, but they haven’t seen that much interest, the AP recently wrote. And the cells can’t be processed like plasma, so it’s unclear what the risk might be from paying donors in this nascent market.