A toxin (Gk. toxikon "(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.
Biotoxins are used in nature for two primary reasons:
- Predation (spider, snake, jellyfish, wasp)
- Defense (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:
- Vipers, such as rattlesnakes.
- Lectin-containing organisms, such as certain plants and microbes.
- 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.
- The box jellyfish.
- Elapid snakes.
- The Cone Snail.
When toxins are generated by bacteria, they are called toxoids. The toxins may be produced either in the living body during infection (for instance, by tetanus) or by bacteria in dead biological material.
In the context of complementary medicine the term is often used as a broader category of any harmful substance claimed to cause ill health.