toxin (redirected from intracellular toxin)

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.

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toxin

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. See also antidote; food poisoning.

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toxin

1. any of various poisonous substances produced by microorganisms that stimulate the production of neutralizing substances (antitoxins) in the body

2. any other poisonous substance of plant or animal origin

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Toxin

Properly, a poisonous protein, especially of bacterial origin. However, nonproteinaceous poisons, such as fungal aflatoxins and plant alkaloids, are often called toxins. See Aflatoxin, Alkaloid

Bacterial exotoxins are proteins of disease-causing bacteria that are usually secreted and have deleterious effects. Several hundred are known. In some extreme cases a single toxin accounts for the principal symptoms of a disease, such as diphtheria, tetanus, and cholera. Bacteria that cause local infections with pus often produce many toxins that affect the tissues around the infection site or are distributed to remote organs by the blood. See Cholera, Diphtheria, Staphylococcus

Toxins may assist the parent bacteria to combat host defense systems, to increase the supply of certain nutrients such as iron, to invade cells or tissues, or to spread between hosts. Sometimes the damage suffered by the host organism has no obvious benefit to the bacteria. For example, botulinal neurotoxin in spoiled food may kill the person or animal that eats it long after the parent bacteria have died. In such situations it is assumed that the bacteria benefit from the toxin in some other habitat and that the damage to vertebrates is accidental. See Food poisoning

Certain bacterial and plant toxins have the unusual ability to catalyze chemical reactions inside animal cells. Such toxins are always composed of two functionally distinct parts termed A and B, and they are often called A-B toxins. The B part binds to receptor molecules on the animal cell surface and positions the toxin upon the cell membrane. Subsequently, the enzymically active A portion of the toxin crosses the animal cell membrane and catalyzes some intracellular chemical reaction that disrupts the cell physiology or causes cell death. See Immunologic cytotoxicity

A large group of toxins breach the normal barrier to free movement of molecules across cell membranes. In sufficient concentration such cytolytic toxins cause cytolysis, a process by which soluble molecules leak out of cells, but in lower concentration they may cause less obvious damage to the cell's plasma membrane or to its internal membranes. See Cell membranes, Cell permeability

Tetanus and botulinal neurotoxins block the transmission of nerve impulses across synapses. Tetanus toxin blockage results in spastic paralysis, in which opposing muscles contract simultaneously. The botulinal neurotoxins principally paralyze neuromuscular junctions and cause flaccid paralysis.

Gram-negative bacteria, such as Salmonella and Hemophilus, have a toxic component in their cell walls known as endotoxin or lipopolysaccharide. Among other detrimental effects, endotoxins cause white blood cells to produce interleukin-1, a hormone responsible for fever, malaise, headache, muscle aches, and other nonspecific consequences of infection. The exotoxins of toxic shock syndrome and of scarlet fever induce interleukin-1 and also tumor necrosis factor, which has similar effects. See Endotoxin, Scarlet fever, Toxic shock syndrome

Toxoids are toxins that have been exposed to formaldehyde or other chemicals that destroy their toxicities without impairing immunogenicity. When injected into humans, toxoids elicit specific antibodies known as antitoxins that neutralize circulating toxins. Such immunization (vaccination) is very effective for systemic toxinoses, such as diphtheria and tetanus. See Antibody, Immunity, Vaccination