CLEANLINESS IS BAD. Most of the cells in your body are not yours. Given half a chance your body will try to kill you.

In An Elegant Defense: The Extraordinary New Science of the Immune System: A Tale in Four Lives, Matt Richtel explains at length, and with verve, how today’s fastest changing and most important branch of medical research has overturned what we think we know about ourselves and our environment.

Having won a Pulitzer Prize for a series of articles on distracted driving that demystified the science of attention, Richtel has exactly the right set of tools to explain how everything you know is wrong, why it’s wrong, and why that’s important — as well as what we actually now believe is right. Not only does he make all this accessible, he also ensures that you don’t feel bad about your previous misconceptions; after all, you’re no different from most scientists until the very recent past.

He knows, scientifically, that to keep our attention he needs to tell stories about people. An Elegant Defense tells the story of four individuals whose immune systems were the key to their health — or to their illness. Richtel uses them as a starting point to explore, in clear terms, the history of the research into the immune system and exactly how we now understand it to work. But these individuals aren’t just convenient devices; their stories — especially that of his close childhood friend, Jason Greenstein — are deeply affecting.

Sometimes an immune system, like the one possessed by Robert T. Hoff, is so poised that it can even defeat HIV. At other times, as with Merredith and Linda Segre, it is “a killer inside.” With all its power trained on the wrong targets, it can lead to debilitating symptoms, devastating side effects, and possible death. An “out-of-whack” immune system can see overachievers laid low, champion sportswomen relegated to wheelchairs, and marriages pushed to divorce. But, Richtel notes, that incredible power could, at the same time, be harnessed to cure cancer.

A decade ago, when I wrote a chapter of my PhD thesis on the “Rhetoric of Immunobiology Textbooks,” the story of the immune system was remarkably uniform. It went like this: in the early 18th century, Lady Mary Wortley Montagu traveled beyond “civilized” Europe to the Ottoman Empire and brought back the technique of variolation — a primitive form of vaccination — to the domestic sphere of Britain. Late in the century, Edward Jenner added her insight to his own and, to simplify the next steps, we have vaccination, resistance, immunity.

In this way, the conception of the immune system recapitulated the genesis of its science. According to that story, the immune system needs to defend the domestic, familiar world against the wild, dangerous outside. Occasionally, the system needs tamed doses of that wild outside, in order to help inoculate and defend us against a pathological world. But our resistance to disease is mostly dependent on keeping the outside out.

However, as Richtel shows, we now understand that our health depends not just on a carefully regulated and directed immune system, but also on the vast numbers of nonhuman cells that live inside the body. Even the idea of a simple “inside” and “outside” is wrong and dangerous. For example, the tube of our digestive tract, stretching from mouth to anus, is, topologically speaking, a long surface of the outside stretching through the body. Without accurate information and constructive direction, the powerful immune system that patrols the entire body, with its myriad surfaces, can be fatal. Given the deep, archaic prejudices that, in the face of science, rule the United States’s executive branch, it is difficult to believe that Richtel avoids metaphors of political pathology. Yet he mostly does.

The world that Richtel paints is one where the distinction between the inside and the outside of the body is of complex significance. The internal environment of people’s bodies — bodies that are not just medical sites, but people with stories and lives — is a different playing field from the external one. But our ability to fight disease is not based on the maintenance of borders impermeable to alien cells. As we now know, our own immune system is often pathological and, conversely, alien cells are profoundly beneficial. A healthy individual relies on her microbiome — that collection of hundreds of trillions of massively varied bacteria that live in our bodies in greater numbers and greater diversity than the cells with our own DNA.

Richtel objects to the metaphor of war and surveillance: it is “misleading, incomplete — even arguably dead wrong.” In his view, “life is a raucous festival, your body like a sprawling party, a chaotic and exuberant affair populated with a variety of cells.” There are indeed many threats to this “festival with open seating,” where, given the “porous nature of our bodies’ borders […] [j]ust about every organism that wants to get inside us can do so.” The metaphors he uses are those of a precision team rather than a sledgehammer army, and an elegant defense rather than an undiscriminating barrier: “Your immune system isn’t a war machine. It’s a peacekeeping force that more than anything else seeks to create harmony.”

Above all, Richtel shows that the operations of the immune system are important because of their profound and direct effects on people’s lives. Our new understandings may have implications that extend beyond our physical health, but primarily they can help make populations healthier and cure the people we love.

Other commentators will surely make a strong case that those who deny the centrality of outsiders in maintaining the health of the body politic are in fact doomed to inflict crippling autoimmune syndromes on that body. Richtel, for his part, constructs from these four individual cases a compelling modern history of — as well as an elegant defense for — the preeminent science of our time.

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Dan Friedman is a contributing editor to 8by8Mag.com, a critic, and the author of an ebook about 1980s rock group Tears for Fears.