PHYSICAL BARRIERS Our first line of defense against invaders consists of physical barriers, and to cause real trouble, viruses, bac- teria, parasites, and fungi must first penetrate these shields. Although we tend to think of our skin as the main barrier, the area covered by our skin is only about two square meters. In contrast, the area covered by the mucous membranes that line our digestive, respiratory, and reproductive tracts measures about 400 square meters – an area about as big as two tennis courts. The main point here is that there is a large perimeter which must be defended