Your immune system’s war isn’t Saving Private Ryan — it’s Homeland

Before I read this book, I imagined the immune system as a defensive force, like the Germans on the beaches at Normandy on June 6 1944. When you’re young and vital, your immune system is the Germans in the early morning — scanning the horizon for movement, with plenty of ammunition in reserve. But life is a process of attrition; as you get older, you become like the Germans later that afternoon — your machine guns get jammed up, and then you use rifles, and pistols, and eventually bayonets, until the invaders finally destroy you — just like the first 20 minutes of Saving Private Ryan.

That’s what I used to think. Now I’ve read this book, I see things a bit differently. Of course, I was right in part — our bodies do have a border patrol, the mucosal immune system, which is responsible for more than 300 square metres of vulnerable territory.