Self & Other
5. The Border Towns
Here is a fact about your body that, when you really sit with it, is hard to fit inside your head.
If you took every cell in your body — every skin cell, every muscle cell, every neuron, every red blood cell, every cell in your liver and lungs and bones — and you put them all in one pile, and then you took every bacterium living in and on you and put those in another pile, the second pile would be roughly the same size as the first.
Roughly thirty-eight trillion bacteria live in or on you, against roughly thirty trillion human cells. The exact ratio depends on whom you ask and how they count, and the once-famous “ten times more bacteria than human cells” estimate has been quietly walked back to something closer to one-to-one. But the basic picture stands. By cell count, you are about half you and about half something else.
Which raises the question we asked back in Chapter 1, in a sharper form. If your immune system spends every minute of every day asking “is this me, or is this something else?” — and if half of what’s inside your skin is, by any reasonable definition, something else — why aren’t you constantly inflamed? Why aren’t you, at every moment, in a state of all-out civil war?
The answer is that most of what we have described so far in this book — the macrophages, the neutrophils, the B cells, the T cells, the spectacular machinery of adaptive immunity — is not, actually, where most of your immune system lives. Most of your immune system lives at the borders. And the borders, it turns out, run by different rules than the inside.
This chapter is about those borders. The skin. The gut. The lungs. The thin sheets of tissue that separate the inside of you from the outside of you, and the strange, ongoing, mostly-peaceful negotiation that happens there.
What Counts as Outside
Hold a piece of paper up in front of you. The paper has two sides. They are clearly different sides. The same is true of your skin: there’s an outside, where the world is, and an inside, where you are.
Now imagine that the paper is rolled into a tube. There is still an outside and an inside. But if you look closely at the tube, you’ll notice something: the inside of the tube is, in a strange way, also outside. If you push a marble through the tube, the marble doesn’t really enter the paper — it travels through a hollow space that, while surrounded by paper, was never really part of the paper at all.
Your gut works exactly like this.
When you eat a sandwich, the sandwich is not, in any biologically meaningful sense, inside your body yet. It is inside a long, twisty tube that runs through your body — from your mouth, down your throat, through your stomach, through about twenty-five feet of small intestine, then through five feet of large intestine, and out the other end. For most of that journey, the food is separated from your actual tissues by a thin layer of cells, one cell thick in some places. The food, while in transit, is technically still in the outside world. It just happens to be in the part of the outside world that you have wrapped your body around.
The same is true, more or less, of your lungs. The air you breathe in does not enter your tissue. It sits in branching cavities inside your chest, separated from your blood by a layer of cells so thin you could read newsprint through it. The lungs are not so much an organ as a beautifully folded surface, vast and intimate, where the outside meets you.
If you took your skin, your gut lining, and your lung lining, and laid them all flat, the total area would be somewhere around two hundred square meters. That’s the surface area of a tennis court. Most of that — the overwhelming majority — is not the skin you can see. It’s the gut lining and the lung lining, folded up inside you. The actual visible skin of an adult human is only about two square meters, smaller than a king-size bed.
Almost all of your interaction with the outside world happens at surfaces you cannot see.
The Skin, Briefly
The skin you can see is, for what it’s worth, a remarkable thing. The outer layer is made of cells that have, by the time you encounter them, technically already died. They are flat, hardened, packed with a tough protein called keratin (the same stuff your fingernails are made of), and they are constantly shedding off and being replaced from below. The dust in your house — a substantial fraction of it — is your skin. You replace your entire outer layer every two to four weeks.
This dead, flaking outer layer is your first defense. Bacteria can sit on it, but they have a hard time getting through. The skin’s surface is acidic, around pH 5, which most bacteria find inhospitable. It is dry, which most bacteria find worse. It is patrolled by oils and antimicrobial peptides — small natural antibiotics that your skin cells produce constantly. It is also, importantly, populated by its own bacteria, in vast numbers, which we’ll get to.