Residents of ancient cities built defensive walls to ward off invaders. The city wall acted both as a bulwark to stop the enemy and as a rampart from which defenders could protect the city. Your body is likewise endowed with a protective 'wall'-your skin. How does your skin protect you from intruders?

The surface of your skin teems with bacteria and other microorganisms, some of which can cause infection and disease. Your skin apparently does more than act as a passive barrier. It also seems to ward off invaders by producing antimicrobial proteins, or peptides, that act as defenders. Some of these are on duty constantly. Others line up when the skin is damaged.

The first two groups of antimicrobial peptides discovered, called defensins and cathelicidins, proved to be defenders on call. Both groups are secreted by cells in the upper skin in response to injuries or inflammation. They kill invaders by punching holes in their cell membranes.

In 2001 a team of researchers at the University of Tübingen, Germany, found another type of germ-fighting protein, called dermicidin, which is active all the time. Unlike the other two groups, dermicidin is produced by healthy skin, in the sweat glands. How this protein works is yet unknown. But the fact that perspiration helps ward off disease might explain why people who wash excessively are more prone to skin infections and eczema.

Like an ancient city wall, our skin is a barrier against enemy invaders. Surely, you would agree with the psalmist who said: 'How many your works are, O Jehovah! All of them in wisdom you have made.'-Psalm 104:24.