Excerpt from The Body by Bill Bryson, plus links to reviews, author biography & more

The beginning of a modern understanding of blood can perhaps be said to date from 1900 and an astute discovery by a young medical researcher in Vienna. Karl Landsteiner noticed that when blood from different people was mixed together, sometimes it clumped and sometimes it did not. By noting which samples joined with which others, he was able to divide the samples into three groups, which he labeled A, B, and 0. Although everybody reads and pronounces the last group as the letter O, Landsteiner in fact meant it to be taken as a zero, because it didn't clump at all. Two other researchers at Landsteiner's lab subsequently discovered a fourth group, which they called AB, and Landsteiner himself, forty years later, co-discovered Rh factor—short for "rhesus," from the type of monkey in which it was found.* The discovery of blood types explained why transfusions often failed: because the donor and the recipient had incompatible types. It was a hugely significant discovery, but unfortunately almost no one paid any attention to it at the time. Thirty years would pass before Landsteiner's contribution to medical science was recognized with a Nobel Prize in 1930.

The way blood typing works is this: All blood cells are the same inside, but the outsides are covered with different kinds of antigens—that is, proteins that project outward from the cell surface—and that is what accounts for blood types. There are some four hundred kinds of antigens altogether, but only a few have an important effect on transfusion, which is why we have all heard of types A, B, AB, and O, but not, say, Kell, Giblett, and type E, to name just a very few among many. People with blood type A can donate to those with A or AB but not B; people with B can donate to B or AB but not A; people with AB can donate only to other people with AB blood. People with type O blood can donate to all others, and so are known as universal donors. Type A cells have A antigen on their surface, type B have B, and type AB have both A and B. Put A type blood in a B type person and the recipient body sees it as an invasion and attacks the new blood.

We don't actually know why blood types exist at all. Partly it may be because there simply wasn't any reason for them not to. That is to say, there was no reason to suppose that any person's blood would ever end up in someone else's body, so no reason to evolve mechanisms to deal with such issues. At the same time, by favoring certain antigens in our blood, we can gain improved resistance against particular diseases—though often at a price. People with O blood, for instance, are more resistant to malaria but less resistant to cholera. By developing a variety of blood types and spreading them around among populations, we benefit the species, if not always the individuals within it.

Blood typing had a second, unanticipated benefit: establishing parenthood. In a famous case in Chicago in 1930, two sets of parents, the Bambergers and the Watkinses, had babies in the same hospital  at the same time. After returning home, they discovered to their dismay that their babies were wearing labels with the other family's name on them. The question became whether the mothers had been sent home with the wrong babies or with the right babies mislabeled. Weeks of uncertainty followed, and in the meantime both sets of parents did what parents naturally do: they fell in love with the babies in their care. Finally, an authority from Northwestern University with a name that might have come out of a Marx Brothers movie, Professor Hamilton Fishback, was called in, and he administered blood tests to all four parents, which at the time seemed the very height of technical sophistication. Fishback's tests showed that both Mr. and Mrs. Watkins had type O blood and therefore could produce only a type O baby, whereas the child in their nursery was type AB. So, thanks to medical science, the babies were swapped back to the right parents, though not without a lot of heartache.

Excerpted from The Body by Bill Bryson. Copyright © 2019 by Bill Bryson. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.