why is egg white used as antidote for lead or mercury poisoning?

Answer 1

Egg white isn't an ANTIDOTE for lead posioning, it is a PROPHYLACTIC intended to PREVENT lead poisoning.

Lead posioning occurs when lead enters the bloodstream and binds to proteins in the body. Egg white is itself a protein and if it is swallowed soon after lead is swallowed the lead will bind to the egg protein in the gut and be harmlessly expelled.

It isn't an antidote since it won't help one little bit once someone has lead posioning. It is a prophylactic, a treatment designed to prevent someone developing poisoning.

Egg white also won't help one bit if the source of exposure is through skin conatct or fumes rather than through swallowing.
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All heavy metals coagulate protein .This causes extensive cell damage.Egg white which is mostly protein gives lead alternative to act on.So egg white is coagulated instead of body protein.This is just a substitution mechanism and works till lead remains in digestive tract.
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Alkaline products can be treated by egg whites and water.
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Egg white contains certain amounts of cystein, with a free SH-group able to precipitate lead as sulphide. It has a chance to react with ingested lead before that is taken up in the blood, where it will react with an enzyme used for breathing which has a heterocyclic ring with two neighbouring S-atoms which can only too easily be convinced to form a chelate around a heavy metal ion (Pb, Hg, As, Ag, Sb.) that forms insoluble sulphides.

The lead still will be taken up and deposited in the body, but the immediate symptoms will be lessened.
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