Physiological defence against xenobiotics at their portals of entry to the organism.

The size and relatively high enzyme activity of the liver means that it has the major role in the biotransformation of xenobiotics in mammals. The tissues through which xenobiotics enter the body, lungs, skin, and gastrointestinal tract, have, however, quite a significant capacity for biotransformation, too. Major fractions of small doses of xenobiotics are therefore already metabolized and inactivated before the compounds have penetrated into the body. Most xenobiotics appear in the food and fluid ingested. This emphasises the importance of the gastrointestinal mucous membrane as the first line of defence of body against foreign chemicals. The biotransformation capacity varies with age and environmental chemical exposure, and depends on species and strain.