If you were going to design the perfect immune system, what
would you do? This question is often posed to beginning
immunology students, and the best answer may be so obvious that
it doesn’t occur to most. The best immune system is one that
prevent pathogens from ever gaining access to your squishy bits
in the first place. And so we have barriers – lots of them.
Unfortunately, the best barriers are not always practical.
Plants have rigid cell walls that are almost impervious to
pathogens, but plants don’t need to walk around. We trade that
in for skin and that does pretty well, but it has limitations.
We need to be able to consume tons of nutrients to fuel our
metabolism, and those nutrients need to be absorbed into our
bloodstream. Being permeable to nutrients and water also allows
pathogens a back-door (no pun here – move on) for bacteria and
viruses to get past the gate-keepers. The epithelial cells that
line our guts provide a pretty good physical barrier themselves,
but they’re no match for pathogens. In