toxin-antitoxin, mixture of a poisonous substance, or toxin, with an antitoxin, or antibody, in such proportion that a large percentage of the toxin is neutralized by the antitoxin. Although formerly used to immunize an individual against disease, toxin-antitoxin mixtures have been largely supplanted by toxoids. See immunity.

toxin, poison produced by living organisms. Toxins are classified as either exotoxins or endotoxins. Exotoxins are a diverse group of soluble proteins released into the surrounding tissue by living bacterial cells. Exotoxins have specific reaction sites in the host; e.g., tetanus and botulinum exotoxins affect nerve tissue, and streptococcal toxins attack vascular tissue. Plants and animals also produce protein toxins. Some, such as cobra venom, are enzymes that destroy substances in host tissue. Endotoxins are polysaccharide and phospholipid substances found in the cell walls of bacteria that are freed when the cells die and break up. The pathologic effects of endotoxins, similar for all bacterial sources, include fever, shock, and intestinal hemorrhage. In sufficiently low doses toxins stimulate the production of antibodies, or antitoxins, in the host, and toxins of a specific bacterial species have been injected to elicit formation of antibodies against the disease caused by the bacteria. Toxoids are protein toxins that have been heated or chemically treated to deprive them of their toxicity but not of the ability to induce the formation of antibodies. See venom.

Any substance poisonous to an organism; often restricted to poisons produced by living organisms. In addition to those from such microorganisms as bacteria (see bacterial diseases), dinoflagellates, and algae, there are toxins in fungi (mycotoxins; see aflatoxin; mushroom poisoning), higher plants (phytotoxins), and animals (zootoxins, or venoms). The plants include nightshade (see nightshade family), poison hemlock, foxglove, mistletoe, and poison ivy. Many plant toxins (e.g., pyrethrins, nicotine, rotenone) apparently protect their producers against certain animals (especially insects) or fungi. Similar defensive secretions in animals may be widely distributed or concentrated in certain tissues, often with some sort of delivery system (e.g., spines, fangs). Animals such as spiders and snakes use venoms to catch prey and often for defense. Many normally edible fishes and shellfishes become poisonous after feeding on toxic plants or algae. Seealso antidote; food poisoning.

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