An antidote is a substance which can counteract a form of poisoning. The term ultimately derives from the Greek αντιδιδοναι antididonai, "given against".
The antidotes for some particular toxins are manufactured by injecting the toxin into an animal in small doses and extracting the resulting antibodies from the host animals' blood. This results in an antivenom that can be used to counteract poison produced by certain species of snakes, spiders, and other venomous animals. A number of venoms lack a viable antivenom, and a bite or sting from an animal producing such a toxin often results in death. Some animal venoms, especially those produced by arthropods (e.g. certain spiders, scorpions, bees, etc.) are only potentially lethal when they provoke allergic reactions and induce anaphylactic shock; as such, there is no "antidote" for these venoms because it is not a form of poisoning and anaphylactic shock can be treated (e.g., by the use of epinephrine).
Some other toxins have no known antidote. For example, the poison aconitine, a highly poisonous alkaloid derived from various aconite species has no antidote, and as a result is often fatal if it enters the human body in sufficient quantities.
Ingested poisons are frequently treated by the oral administration of activated charcoal, which adsorbs the poison and flushes it from the digestive tract, thereby removing a large part of the toxin. Poisons which are injected into the body (such as those from bites or stings from venomous animals) are usually treated by the use of a constriction band which limits the flow of lymph and/or blood to the area, thus slowing circulation of the poison around the body. This should not be confused with use of a tourniquet which cuts off blood flow completely - often leading to the loss of the limb.