This is a very important concept in toxicology. Encompasses the differences in susceptibility to toxic effects between different species of animal or plant or between different cells, such as between tumour cells and normal cells.
It is in many cases a useful attribute which is utilized in the design of antibacterial drugs, pesticides or anti-cancer drugs. It is also of relevance to the prediction of toxicity in humans based on studies in another species.
The reasons for selective toxicity are various but can be divided into those due to differences in the absorption. Distribution, metabolism and excretion of a chemical (toxicokinetics) or those due to biochemical differences affecting the presence of a receptor or target molecule (tox-icodynamics).
For example the insect is more susceptible to the toxicity of DDT than mammalian organisms for two reasons. Firstly, the insect cuticle allows DDT to penetrate more readily than the mammalian skin. Secondly, the insect has a greater surface area to volume ratio and therefore absorbs relatively more DDT. Insects are more susceptible to some organophosphorus insecticides because the compound is metabolite oxidative desulphuration to a compound that inhibits acetylcholinesterase, whereas in mammals enzymatic hydrolysis produces a metabolite that is more readily excreted but is not an inhibitor of acetylcholinesterase.
The rodenticide norbormide is against rats because they possess smooth muscle whereas humans, cats and dogs no not. Other rodenticides are based more sim ply on the fact that the rat does not have a reflex, unlike many other mammals. Therefore after the oral ingestion of a poisonous chemical the rat is unable to rid itself of the substance by simply vomiting.
Penicillin is active against certain bacteria because it interferes with synthesis of the wall in multiplying bacteria but mammalian cells do not have a cell wall and therefore are not affected.