Self & Other

Prologue

The Cut on Your Knee

You’re maybe seven years old. It’s a Saturday. You’re riding a bike that’s almost too big for you, and you take the corner too fast on a sidewalk where someone has, very rudely, left a tiny crack. The front wheel jerks. You go down.

Your knee meets the concrete with the particular sound that knees and concrete make when they meet — a sort of papery scrape. You stand up. There’s a moment of confusion before the pain lands. Then, slowly, a bead of blood. Then more. Your sock is going to be a mess.

This is, biologically speaking, an emergency.

Not for you. You’re fine. You’ll be inside in two minutes, someone who loves you will produce the brown bottle of stinging stuff that lives in every house, and by Monday there will be a scab the size of a quarter that you will pick at against medical advice. By the following Saturday you’ll have forgotten about it entirely.

But for a few square millimeters of tissue near your kneecap, the scrape is a catastrophe. A wall has been breached. The outside has gotten in. And in the ninety seconds between the fall and your standing back up, somewhere on the order of a hundred million tiny things happen, in a sequence so fast and well-coordinated that if you tried to write it as a screenplay it would feel like the whole plot of an action movie compressed into a minute and a half.

Skin cells at the edge of the wound start spilling chemicals into the surrounding tissue — alarm signals, basically. Tiny capillaries widen, which is why the area starts to feel warm and look pink. Plasma leaks out from the blood into the tissue, which is what makes the skin around the cut puffy. Within seconds, the first responders show up: cells called neutrophils, which behave a little like a kindergarten class that has been told there is a fire and is running toward it with fire extinguishers, fearless and slightly chaotic. Behind them come the macrophages — bigger, slower, more strategic, the firefighters who actually know what they’re doing.

Meanwhile, on the surface of the bacteria that just got introduced to your bloodstream — bacteria that lived their whole lives on the sidewalk and had no plans to ever meet a human — a different kind of process is starting. Tiny proteins called complement are latching on, drilling holes in some bacteria outright and tagging others like bags at an airport so that other cells know to grab them. A dendritic cell, weird and starfish-shaped, is gathering up bits of bacterial protein and beginning a long, urgent journey to the nearest lymph node, where it will hold up its samples to a passing T cell and ask, in essence: do you recognize this? Does anyone recognize this? If one does, a much larger and stranger cascade begins — one we’ll spend half this book getting to know.

You did not, of course, ask for any of this. You did not consent to it, learn it, or earn it. You’re seven. You’re trying to find your mom.

But this is what your body is doing all the time, mostly without telling you. Every minute of every day, in tissues and bloodstream and gut lining and the inside of your nose, an enormous distributed system is asking the same question, over and over: is this me, or is this something else? When the answer is “me,” nothing happens. When the answer is “something else,” a response begins — sometimes a tiny one you’ll never notice, sometimes a roaring one that puts you in bed for a week.

This is the immune system. And the wonderful, slightly dizzying thing about it is that almost none of it works the way most people think it does.

It is not a wall. It is not an army. It is not, despite what the supplement aisle tells you, a thing you can “boost” by drinking a particular kind of juice. It is closer to a city than a fortress — a complicated, gossipy, mostly-cooperative city full of specialists who mostly know their jobs, occasionally argue, very occasionally stage a riot, and somehow keep the whole place running for, on average, about eighty years.

This book is a tour of that city.

We’ll meet the cast, learn the rules they follow (and the ones they break), and watch what happens when things go beautifully right and when they go painfully wrong. We’ll talk about why you only get chickenpox once. Why allergies are, in a strange way, the immune system being too good at its job. Why some people’s immune systems mistake their own pancreas, or joints, or gut lining, for an invader — and what that means for the people who live in those bodies. We’ll talk about vaccines. About cancer. About the strange and wonderful fact that the bacteria living in your gut outnumber your own cells. We’ll talk about what actually helps you stay well, and what only sounds like it does.

And we’ll come back, at the very end, to that seven-year-old on the sidewalk. Because by the time you’ve finished the book, the ninety seconds after the fall will look very different to you. Not less amazing. More.

Let’s go inside.

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