Unwanted metals (which are usually toxic) can be removed from the body by chelation therapy. Chelation therapy was developed during the First World War, to provide an antidote to the poison gas Δ-chlorovinyldichloroarsine, known as Lewisite.
Lewisite acts by reacting with the SH groups (usually cysteine residues) of various enzymes. The reaction renders the enzymes inactive, and the victim dies.
British Anti-Lewisite (BAL) was developed as an antidote.
Arsenic forms a stronger complex with BAL than with the enzymes, so the ,metal can be removed and the patient lives.
Chelation therapy is now used to remove toxic heavy metals, such as mercury, lead and cadmium, form the body. BAL can be used for mercury and lead poisoning, but not for cadmium poisoning as it forms a soluble complex that can disperse throughout the body.
Chelating agents must be highly selective - they must only remove unwanted metals, not those which are beneficial. For example, the disodium salt of EDTA can be used to treat lead poisoning as it removes lead and has no detrimental effects. The dicalcium salt of EDTA, however, cannot be used as it also removes beneficial zinc.
Other examples of chelating agents include D-penicillamine, which is used to remove a build-up of copper in the body (caused by Wilson's disease), and desferrioxamine B, which is used to remove unwanted iron. These agents are specific; they only remove the unwanted metals.