Excerpt from Immune by Catherine Carver, plus links to reviews, author biography & more

While the lung escorts invaders out in an orderly fashion, the gut takes a more medieval approach to border control: acid. The discovery of this acid has a rather gruesome history. The story begins in June 1822 on the Island of Michilimackinac in the wilds of Michigan. At the time the lush green island, christened 'the great turtle' by the Ottawa and Chippewa tribes, was the main trading post of the American Fur Company (the brainchild of America's first multimillionaire, John Jacob Astor). It was while standing in line at the Fur Company store that 20-year-old trapper Alexis St Martin was accidentally shot in the gut at close range. The only doctor on the island arrived to a scene worthy of any horror movie: 'a portion of the Lungs as large as a turkey's egg protruding through the external wound, lacerated and burnt'. St Martin also had fractured ribs, a torn diaphragm and a hole in his stomach through which his breakfast was spilling out on to his shirt. His doctor, an army surgeon by the name of Beaumont, thought St Martin had little chance of survival but endeavoured to help him nonetheless. Astoundingly, St Martin began to recover and with the care of Beaumont he slowly became whole again.

Well, almost. The hole in his stomach didn't fully heal, and St Martin declined offers from Beaumont to stitch it shut, leaving him with a small but permanent connection between the outside world and his stomach. This physical quirk changed not only the course of their relationship but also the history of science. It began on 30 May 1823 when Beaumont's writing first betrayed his experimental interest in St Martin. He described administering medication directly through the hole into St Martin's stomach 'as never medicine was administered to man since the creation of the world'. After this novelty Beaumont took Alexis into his home where he worked as a servant and aide. The good doctor suggested his offer was extended for charitable reasons, and Alexis was likely penniless; however some have since suggested he had less moral (and more ambitious) motives. Centuries have passed, making any judgements on the ethics of the situation speculative. However what we do know is that by 1825 the experiments had begun in earnest, moving their relationship firmly into one of scientist and subject. Beaumont would tie pieces of food to a silk string and introduce them directly through the hole in Alexis' stomach. He tried a range of foods including cabbage, stale bread and salt beef, and also extracted fluid from St Martin's stomach for further analysis.

Over the course of eight years Beaumont conducted 238 experiments on Alexis, eventually publishing the results in a seminal text called Experiments and Observations on the Gastric Juice and the Physiology of Digestion. Never before had someone had direct access to the inner workings of a human stomach. According to the founding father of medicine, Dr William Osler, it was Beaumont's research that confirmed that hydrochloric acid was the most important acid in the stomach.

Consequently Beaumont became famous. Alexis St Martin also lived a full life and fathered 20 children before eventually dying at the grand old age of 78. However, while his experiences with the medical profession may have enabled him to live, the actions of his family on his death underline the impact the experience had on him and them. When Alexis died his family refused to have his body buried for four days to ensure it would be too decayed for an autopsy. Aware of the tenacity of the medical profession, and its occasional proclivity for grave robbing, they then buried him under over half a metre (2ft) of rock and almost two metres (6ft) of soil. They expressed the strength of their sentiment through a simple telegram sent to the great Dr Osler himself: 'Don't come for autopsy, will be killed.'

In the years since then, scientists have built on Beaumont's work (with rather more mundane methods) and developed an in-depth understanding of the physiology of stomach or 'gastric' acid. Parietal cells dotted around the surface of the stomach are equipped with proton pumps, which are like tiny merry-go-rounds for ions. They take potassium ions out of the stomach juice and drop them off inside the parietal cell. In a neat little dance of exchange they also pick up hydrogen ions from inside the parietal cell and release them into the maelstrom of the stomach. These hydrogen ions swirl around in the stomach contents and go on to combine with chloride ions to make hydrochloric acid. This acid is the reason the pH of the normal stomach is an unwelcoming pH 2, capable of disintegrating many of the bacteria that land in it.

Excerpted from Immune by Catherine Carver. Copyright © 2017 by Catherine Carver. Excerpted by permission of Bloomsbury USA. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.