THE SKIN
The skin is like the wall around an ancient city, a formi- dable protective barrier. It not only prevents the entry of parasites but also protects against injury by mechani- cal, thermal, and chemical forces. Unlike induced
defenses such as fever, which are aroused only when a particular dan- ger threatens, the skin is constantly present, always on guard. It is tough and much more resistant to puncture and abrasion than the internal tissues it protects. Minor infections here and there are harm- less because the skin is constantly being sloughed off the top and renewed from below. An ink stain on the fingers will be gone in a few days, not because the ink has been absorbed or chemically altered but because the stained cells are replaced by others rising from below. Fungal growths or other potential pathogens in surface cells are con- stantly cast off by this rapid replacement of the epidermis. Sycamores and shagbark hickory trees seem to use the same strategy.
Not only is the skin a good defensive armor in general, it is also good in particular. Those parts of the body that are most in need of armor, such as the soles of the feet, have thicker and tougher skin right from birth. Any particular patch of skin that is subjected to repeated friction, like that at the top edge of a shoe or the tip of a cel- list’s finger, grows the thicker skin we call a callus. This adaptive growth, an induced defense, not only minimizes mechanical injury, it also prevents breaks in the skin that could provide entrances for pathogens.
