After last year’s Boston Marathon bombings, many people rushed to donate blood—so much that the Red Cross, in a statement the next day, said it had more than it needed. An adequate blood supply is something most of us now take for granted: Thanks to altruistic donors, we assume that, if we end up in the hospital after a traffic accident or for open-heart surgery, the blood will be there.

This was not always the case. In the early years of blood transfusion, patients were often out of luck if they couldn’t find—or couldn’t afford—a paid blood donor. That began to change in the 1930s, when a Chicago hospital set up the first “blood bank,” a term that once really did run on the idea that blood could be repaid with blood, much as we might pay a cash loan back with money. Those who received blood were asked to come back to the hospital once they recovered (or send a relative in their place) to replenish the bank with a donation. Later, blood banks began to rely more and more on volunteer donors, who deposit the blood that hospitals store and patients rely on.

The enormous success of volunteer blood banking has shaped the way we now think about body products in general, writes Northeastern University law professor Kara Swanson in a new history, “Banking on the Body: The Market in Blood, Milk, and Sperm in Modern America” (Harvard University Press). Today, we divide body products into “commodities” and “gifts”—roughly speaking, options and necessities. Sperm and eggs are perfectly legal to sell and buy, while lifesaving products like bone marrow and organs follow the blood model, and are considered beyond the market.