Toxin

A toxin (Greek:, toxikon, lit. (poison) for use on arrows) is a poisonous substance produced by living cells or organisms. Toxins are nearly always proteins that are capable of causing disease on contact or absorption with body tissues by interacting with biological macromolecules such as enzymes or cellular receptors. Toxins vary greatly in their severity, ranging from usually minor and acute (as in a bee sting) to almost immediately deadly (as in botulinum toxin).

Biotoxins vary greatly in purpose and mechanism, and can be highly complex (the venom of the cone snail contains dozens of small proteins, each targeting a specific nerve channel or receptor), or relatively small protein.

Use
Biotoxins in nature have two primary functions:
 * Predation (spider, snake, scorpion, jellyfish, wasp)
 * Defense (bee, poison dart frog, deadly nightshade, honeybee, wasp)

Some of the more well known types of biotoxins include:
 * Hemotoxins target and destroy red blood cells, and are transmitted through the bloodstream. Organisms that possess hemotoxins include:
 * Pit Vipers, such as rattlesnakes.
 * Necrotoxins cause necrosis (i.e., death) in the cells they encounter and destroy all types of tissue. Necrotoxins spread through the bloodstream, but infect all tissues.  In humans, skin and muscle tissues are most sensitive to necrotoxins.  Organisms that possess necrotoxins include:
 * The brown recluse or "fiddle back" spider.
 * Necrotizing fasciitis (the "flesh eating" bacteria)
 * Neurotoxins primarily affect the nervous systems of animals. Organisms that possess neurotoxins include:
 * The Black Widow and other widow spiders.
 * Most scorpions.
 * The box jellyfish.
 * Elapid snakes.
 * The Cone Snail.

Plant Toxins

Ricin is found in the castor bean plant.

Non-technical usage
When used non-technically, the term "toxin" is often applied to any toxic substances. Toxic substances not of biological origin are more properly termed poisons. Many non-technical and lifestyle journalists also follow this usage to refer to toxic substances in general, though some specialist journalists at publishers such as BBC and The Guardian maintain the distinction that toxins are only those produced by living organisms.

In the context of alternative medicine the term is often used nonspecifically to refer to any substance claimed to cause ill health, ranging anywhere from trace amounts of pesticides to common food items like refined sugar or additives like artificial sweeteners and MSG.

The term is also used commonly in pop psychology to describe things that have an adverse effect on psychological health, such as a "toxic relationship," "toxic work environment" or "toxic shame."