Antivenom (or antivenin or antivenene) is a biological product used in the treatment of venomous bites or stings. Antivenom is created by milking venom from the desired snake, spider or insect. The venom is then diluted and injected into a horse, sheep or goat. The subject animal will undergo 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. Internationally, antivenoms must conform to the standards of pharmacopoeia and the World Health Organization (WHO).
Historically, the term antivenin was predominant around the world, its first published use being in 1895. In 1981, the World Health Organization decided that the preferred terminology in the English language would be venom and antivenom rather than venin and antivenin or venen and antivenene.
Therapeutic use 
The principle of antivenom is based on that of vaccines, developed by Edward Jenner; however, instead of inducing immunity in the patient directly, it is induced in a host animal and the hyperimmunized serum is transfused into the patient.
Antivenoms can be classified into monovalent (when they are effective against a given species' venom) or polyvalent (when they are effective against a range of species, or several different species at the same time). The first antivenom for snakes (called an anti-ophidic serum) was developed by Albert Calmette, a French scientist of the Pasteur Institute working at its Indochine branch in 1895, against the Indian Cobra (Naja naja). Vital Brazil, a Brazilian scientist, developed in 1901 the first monovalent and polyvalent antivenoms for Central and South American Crotalus, Bothrops and Elaps genera, as well as for certain species of venomous spiders, scorpions, and frogs. They were all developed in a Brazilian institution, the Instituto Butantan, located in São Paulo, Brazil.
Antivenoms for therapeutic use are often preserved as freeze-dried ampoules, but some are available only in liquid form and must be kept refrigerated. They are not immediately inactivated by heat, however, so a minor gap in the cold chain is not disastrous. The majority of antivenoms (including all snake antivenoms) are administered intravenously; however, stonefish and redback spider antivenoms are given intramuscularly. The intramuscular route has been questioned in some situations as not uniformly effective.
Antivenoms bind to and neutralize the venom, halting further damage, but do not reverse damage already done. Thus, they should be administered as soon as possible after the venom has been injected, but are of some benefit as long as venom is present in the body. Since the advent of antivenoms, some bites which were previously invariably fatal have become only rarely fatal provided that the antivenom is administered soon enough.
Antivenoms are purified by several processes but will still contain other serum proteins that can act as antigens. Some individuals may react to the antivenom with an immediate hypersensitivity reaction (anaphylaxis) or a delayed hypersensitivity (serum sickness) reaction and antivenom should, therefore, be used with caution. Despite this caution, antivenom is typically the sole effective treatment for a life-threatening condition, and once the precautions for managing these reactions are in place, an anaphylactoid reaction is not grounds to refuse to give antivenom if otherwise indicated. Although it is a popular myth that a person allergic to horses "cannot" be given antivenom, the side effects are manageable, and antivenom should be given as rapidly as the side effects can be managed.
In the U.S. the only approved antivenom for pit viper (rattlesnake, copperhead and water moccasin) snakebite is based on a purified product made in sheep known as CroFab. It was approved by the FDA in October, 2000. U.S. coral snake antivenom is no longer manufactured, and remaining stocks of in-date antivenom for coral snakebite expired in the Fall of 2009, leaving the U.S. without a coral snake antivenom. Efforts are being made to obtain approval for a coral snake antivenom produced in Mexico which would work against U.S. coral snakebite, but such approval remains speculative. In the absence of antivenom, all coral snakebite should be treated in a hospital by elective endotracheal intubation and mechanical ventilation until the effects of coral snake neurotoxins abate. It is important to remember that respiratory paralysis in coral snakebite can occur suddenly, often up to 12 or more hours after the bite, so intubation and ventilation should be employed in anticipation of respiratory failure and not after it occurs, when it may be too late.
Natural and acquired immunity 
Although individuals can vary in their physiopathological response and sensitivity to animal venoms, there is no natural immunity to them in humans. Some ophiophagic animals are immune to the venoms produced by some species of venomous snakes, by the presence of antihemorrhagic and antineurotoxic factors in their blood. These animals include Kingsnakes, opossums, mongooses, ( which naturally hunt and kill venomous snakes such as the King Cobra) and hedgehogs.
It is quite possible to immunize a person directly with small and graded doses of venom rather than an animal. According to Greek history, King Mithridates did this in order to protect himself against attempts of poisoning, therefore this procedure is often called mithridatization. However, unlike a vaccination against disease which must only produce a latent immunity that can be roused in case of infection, to neutralize a sudden and large dose of venom requires maintaining a high level of circulating antibody (a hyperimmunized state), through repeated venom injections (typically every 21 days). The long-term health effects of this process have not been studied. For some large snakes, the total amount of antibody it is possible to maintain in one human being is not enough to neutralize one envenomation. Further, cytotoxic venom components can cause pain and scarring at the immunization site. Finally, the resistance is specific to the particular venom used; maintaining resistance to a variety of venoms requires multiple monthly venom injections. Thus, there is no practical purpose or favorable cost/benefit ratio for this, except for people like zoo handlers, researchers, and circus artists who deal closely with venomous animals. Mithridatization has been tried with success in Australia and Brazil and total immunity has been achieved even to multiple bites of extremely venomous cobras and pit vipers. Starting in 1950, Bill Haast successfully immunized himself to the venoms of Cape, Indian and King cobras.
Because neurotoxic venoms must travel farther in the body to do harm and are produced in smaller quantities, it is easier to develop resistance to them than directly cytotoxic venoms (such as those of most vipers) that are injected in large quantity and do damage immediately upon injection.
Availability of antivenoms 
Antivenoms have been developed for the venoms associated with the following animals:
|Funnel web spider antivenom||Sydney funnel-web spider||Australia|
|Soro antiaracnidico||Brazilian wandering spider||Brazil|
|Soro antiloxoscelico||Recluse spider||Brazil|
|Suero antiloxoscelico||Chilean recluse||Peru|
|Aracmyn||All species of Loxosceles and Latrodectus||Mexico|
|Redback spider antivenom||Redback spider||Australia|
|Black widow antivenom||Black widow spider||United States|
|SAIMR Spider antivenom||Button spider||South Africa|
|Anti Latrodectus antivenom||Black Widow spider||Argentina|
|Tick antivenom||Paralysis tick||Australia|
|Soro antilonomico||Lonomia oblique caterpillar||Brazil|
|Alacramyn||Centruroides limpidus, C. noxius, C. suffusus||Mexico|
|Suero Antialacran||Centruroides limpidus, C. noxius, C. suffusus||Mexico|
|Tunisian polyvalent antivenom||All Iranian scorpions||Tunisia|
|Anti-Scorpion Venom Serum I.P.(AScVS)||Indian red scorpion||India|
|Anti-scorpionique||Androctonus spp., Buthus spp.||Algeria|
|Scorpion antivenom||Black scorpion, Buthus occitanus||Morocco|
|Soto antiabortionist||Titus sop.||Brazil|
|SAMURAI scorpion antivenin||Parachutes sop.||South Africa|
|Purified prevalent Anti-Scorpion Serum(equine)||Leisure sop.& Andirons scorpions||Egypt|
Marine animals 
|CSL box jellyfish antivenom||Box jellyfish||Australia|
|CSL stonefish antivenom||Stonefish||Australia|
|Polyvalent snake antivenom||South American Rattlesnake Crotalus durissus and fer-de-lance Bothrops asper||Mexico (Instituto Bioclon)|
|Polyvalent snake antivenom||South American Rattlesnake Crotalus durissus and fer-de-lance Bothrops asper||South America|
|Polyvalent snake antivenom||Saw-scaled Viper Echis carinatus, Russell's Viper Daboia russelli, Spectacled Cobra Naja naja, Common Krait Bungarus caeruleus||India|
|Death adder antivenom||Death adder||Australia|
|Black snake antivenom||Pseudechis spp.||Australia|
|Tiger snake antivenom||Australian copperheads, Tiger snakes, Pseudechis spp., Rough scaled snake||Australia|
|Brown snake antivenom||Brown snakes||Australia|
|Polyvalent snake antivenom||Many Australian snakes||Australia|
|Sea snake antivenom||Sea snakes||Australia|
|Vipera tab||Vipera spp.||USA|
|Polyvalent crotalid antivenin (CroFab—Crotalidae Polyvalent Immune Fab (Ovine))||North American pit vipers (all rattlesnakes, copperheads, and cottonmouths)||North America|
|Soro antibotropicocrotalico||Pit vipers and rattlesnakes||Brazil|
|SAIMR polyvalent antivenom||Mambas, Cobras, Rinkhalses, Puff adders (Unsuitable small adders: B. worthingtoni, B. atropos, B. caudalis, B. cornuta, B. heraldica, B. inornata, B. peringueyi, B. schneideri, B. xeropaga)||South Africa|
|SAIMR echis antivenom||Saw-scaled vipers||South Africa|
|SAIMR Boomslang antivenom||Boomslang||South Africa|
|Panamerican serum||Coral snakes||Costa Rica|
|Anticoral||Coral snakes||Costa Rica|
|Anti-mipartitus antivenom||Coral snakes||Costa Rica|
|Anticoral monovalent||Coral snakes||Costa Rica|
Antivenom sources 
The following groups assist in locating antivenoms:
- Africa: South African Institute for Medical Research, Johannesburg, Republic of South Africa.
- Australia: CSL Limited, Parkville, Victoria.
- Brazil: Instituto Butantan, São Paulo
- Costa Rica: Instituto Clodomiro Picado, San José
- Mexico: Instituto Bioclon
- Mexico: Veteria Laboratories SA de CV (US subsidiary - BioVeteria Life Sciences, LLC)
- USA, The Antivenom Index is a joint project of the Association of Zoos and Aquariums and the American Association of Poison Control Centers: They maintain a website to help locate rare antivenoms: http://www.aza.org/ai/
- USA, Colorado: Poisindex central office in Denver, Colorado, USA (1-800-332-3073).
- USA, Miami, Florida: The Miami-Dade Fire Rescue Antivenom Bank: Emergency: 1-786-336-6600 available 24 hours. A list of available antivenoms and more information about the bank is available at http://www.miamidade.gov/mdfr/emergency_special_venom.asp
- Theakston RD, Warrell DA, Griffiths E (April 2003). "Report of a WHO workshop on the standardization and control of antivenoms". Toxicon 41 (5): 541–57. doi:10.1016/S0041-0101(02)00393-8. PMID 12676433.
- Merriam-Webster Dictionary Antivenin
- World Health Organization (1981). Progress in the characterization of venoms and standardization of antivenoms. Geneva: WHO Offset Publications. p. 5. ISBN 92-4-170058-0.
- Isbister GK. (2002). "Failure of intramuscular antivenom in Redback spider envenoming". Emerg Med (Fremantle) 14 (4): 436–9. doi:10.1046/j.1442-2026.2002.00356.x. PMID 12534488.
- See, for example, the Antivenom Precautions paragraph of the Medication section of James Forster, MD, MS (2006-03-14). "Snake Envenomations, Sea". eMedicine Emergency Medicine (environmental). Archived from the original on 26 June 2006. Retrieved 2006-06-25.
- http://www.savagelabs.com/Products/CroFab/Home/crofab_frame.htm Link to PDF for full prescribing information, retrieved 11/11/12
- "Appendix: Antivenom Tables". Clinical Toxicology 41 (3): 317–27. 2003. doi:10.1081/CLT-120021117.
- Spawls, S; Branch B (1995). The Dangerous Snakes of Africa. Ralph Curtis Books. Dubai: Oriental Press. p. 192. ISBN 0-88359-029-8.