We always reach for the analogies that match our experience most closely. If this were 200 years ago, we'd say that the immune system was like a lock-and-key me

We always reach for the analogies that match our experience most closely. If this were 200 years ago, we'd say that the immune system was like a lock-and-key mechanism, or perhaps we'd say it was like a bank checking for correct signatures. Indeed, I remember a kid's cartoon about our bodies when I was growing up that showed germs that had evolved the correct antigens as being spies passing through passport control with forged passports.