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A toxin is a toxicant that is produced by a living organism and is not used as a synonym for toxicant—all toxins are toxicants, but not all toxicants are toxins. Toxins, whether produced by animals, plants, insects, or microbes are generally metabolic products that have evolved as defense mechanisms for the purpose of repelling or killing predators or pathogens. The action of natural toxins has long been recognized and understood throughout human history. For example, ancient civilizations used natural toxins for both medicinal (therapeutic) and criminal purposes.
Microbial Toxins : The term 'microbial toxin' is a toxin produced by Microorganisms that are of high molecular weight
and have antigenic properties. They include some of the most toxic substances known, such as tetanus toxin, botulinus toxin, and diphtheria toxin. Bacterial toxins may be extremely toxic to mammals and may affect a variety of organ systems, including the nervous system and the cardiovascular system.  
Mycotoxins :The term 'Mycotoxins' is a toxin produced by Fungi Mycotoxins of most interest are those found in human food or in the feed of domestic animals. They include the ergot alkaloids produced by Claviceps sp., aflatoxins and related compounds produced by Aspergillus sp., The ergot alkaloids are known to affect the nervous system and to be vasoconstrictors. Mycotoxins may also be used for beneficial purposes. The mycotoxin avermectin is currently generating considerable interest both as an insecticide and for the control of nematode parasites of domestic animals.Mycotoxins are believed to result in headaches, sore throats, hair loss, flu symptoms, diarrhea, fatigue, dermatitis, general malaise (tiredness) and psychological depression.'
Algal Toxins : Algal toxins are broadly defined to represent the array chemicals derived from many species of cyanobacteria (blue-green bacteria), dinoflagellates, and diatoms. The toxins produced by these freshwater and marine organisms often accumulate in fish and shellfish inhabiting the surrounding waters, causing both human and animal poisonings, as well as overt fish kills. Unlike many of the microbial toxins, algal toxins are generally heat stable and, therefore, not altered by cooking methods, which increases the likelihood of human exposures and toxicity. Diarrhetic shellfish toxins (DST) cause diarrhoea.
  Phytotoxins: Toxic chemicals produced by plants are phytotoxins. Many of the common drugs of abuse such as cocaine, caffeine, nicotine, morphine, and the cannabinoids are plant toxins.
Animal Toxins : Some species from practically all phyla of animals produce toxins for either offensive or defensive purposes. Some are passively venomous, often following inadvertentingestion, whereas others are actively venomous, injecting poisons through specially adapted stings or mouthparts.Snake venoms have been studied extensively; their effects are due, in general, to toxins that are peptides with 60 to 70 amino acids. These toxins are cardiotoxic or neurotoxic, and their effects are usually accentuated by the phospholipases, peptidases,proteases, and other enzymes present in venoms. These enzymes may affect the blood clotting mechanisms and damage blood vessels.
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