A toxin is a poisonous substance produced within living cells or organisms; man-made substances created by artificial processes are thus excluded. The term was first used by organic chemist Ludwig Brieger (1849–1919). For a toxic substance not produced within living organisms, "toxicant" and "toxics" are also sometimes used.. 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 and acute to almost immediately deadly.