Venom or zootoxin (literally, animal poison) is any of a variety of poisons used by several groups of animal species, for the purpose of defence and hunting prey. Though subjective, the definition of a venom differs from a poison (as used in most zoology and medicine texts). Generally, a venom is defined as a biologic toxin that is injected to cause its effect while a poison is a biologic toxin which is absorbed through epithelial linings (either of the gut or through the skin).
The animals most widely known to use venom are snakes, some species of which inject venom into their prey through hollow fangs, spiders and centipedes, which also inject venom through "fangs," scorpions, and stinging insects, which inject venom with a sting (which is in fact a modified egg-laying device - the ovipositor). There are also many caterpillars that have defensive venom glands associated with specialized bristles on the body, known as urticating hairs, some of which can be lethal to humans (e.g., the Lonomia moth). Venom is also found in other reptiles besides snakes such as the gila monster, mexican beaded lizard, iguana, and various species of monitor lizard. Other insects, such as true bugs , also produce venom.
Venom can also be found in some fishes such as the stonefish, scorpionfish, rabbitfish, weever fish, cowfish, boxfish, sabre-toothed blennies, chimaera fish, weaver fish, squirrelfish, scat fish, stingrays, and some toadfish and catfish; as well as some jellyfish, mollusks, amphibians, and even in a few mammals like the platypus, and some shrews.
Bees use an acidic venom designed to cause pain to the stung, because their purpose is to defend their home and food stores, while wasps use a chemically different venom designed to paralyze the prey, so it can be stored alive in the food chambers of their young. The use of venom is much more widespread than just these examples, of course.
It is important to note the difference between "venomous" and "poisonous", which are two commonly confused terms with regards to plant and animal life. Venomous, as stated above, refers to animals who inject venom into their prey or as a self-defence mechanism while the organism is still alive. Poisonous, on the other hand, refers to plants or animals that are harmful when consumed or touched. One bird species the hooded pitohui, although not venomous, is poisonous, secreting a neurotoxin on to its skin and feathers, as do the primates called slow lorises.
Snake venom is produced by glands below the eye and delivered to the victim through tubular or channeled fangs. Snake poisons contain a variety of peptide toxins. Snakes use their venom principally for hunting, though the threat of being bitten is used for defence. Snake bites cause pain, swelling, tissue damage, low blood pressure and convulsions (according to the species of snake).
Antivenin is used in the treatment of venomous bites. It is created by injecting a small amount of the targeted venom into an animal such as a sheep, horse, goat, or rabbit; the subject animal will suffer an immune response to the venom, producing antibodies against the venom's active molecule which can then be harvested from the animal's blood and used to treat envenomation in others. This treatment may only be used on a given person a certain number of times, however, as that person will develop their own antibodies against the foreign animal antibodies injected into them. Even if that person doesn't have a serious allergic reaction to the antivenin, his or her own immune system can destroy the antivenin before the antivenin can destroy the venom. Though most people never require one treatment of antivenin in their lifetime, let alone several, people who work with snakes or other poisonous animals may. Luckily, these people often develop enough antibodies of their own against the venom of whatever animals they handle to become immune themselves, without needing the help of non-human antibodies.