A toxin is a poisonous substance produced within living cells or organisms; synthetic toxicants created by artificial processes are thus excluded. The term was first used by organic chemist Ludwig Brieger (1849–1919).

Toxins can be small molecules, peptides, or proteins that are capable of causing disease on contact with or absorption by body tissues interacting with biological macromolecules such as enzymes or cellular receptors. Toxins vary greatly in their severity, ranging from usually minor (such as a bee sting) to almost immediately deadly (such as botulinum toxin).

Anatomy of an amiotic egg This article is part of Project Glossary, a All Birds project that aims to write comprehensive articles on each term related to animals.

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