Xenobiotic Protection/Resistance Mechanisms in Organisms

Organisms have always been exposed to chemicals that are foreign to them. Evolution has produced protective mechanisms against natural chemicals, mechanisms that are currently used against exposures to anthropogenically sourced chemical contaminants. Protection can occur at the individual level, often called tolerance, or at the population level, which is called resistance. The earliest studies on tolerance and resistance mechanisms come from the insect/pesticide literature; however recent environmental and health research is providing a wealth of information on these mechanisms in other organisms including humans. There are two major categories of protective mechanisms: (1) toxicokinetically derived mechanisms, which alter the way in which organisms absorb, biotransform, and excrete chemicals; and (2) toxicodynamically derived mechanisms, in which target sites are modified to reduce sensitivity. In both of these categories, protection can occur at the molecular and genetic level, through the cellular/tissue and organ levels, and up to and including whole organism responses including changes in behavior.