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The terms antivenom (English) and antivenin (French and other countries) often are used interchangeably. In 1981, the World Health Organization determined that the preferred terms for the English language are "venom" and "antivenom." Venin is the French word for venom and antivenin is traditionally used in certain parts of the world, but Wyeth, the maker of Crotaline and Micrurus antivenom, and Merck and Company, the makers of Latrodectus antivenom, adopted antivenin in the brand names for their products.

Antivenom for spiders and scorpions is prepared in the same manner as other antivenom products by first immunizing animals with nontoxic amounts of venom.4,35 Monkeys, horses, goats, sheep, chickens, camels, and rabbits have been used as sources of antivenom.40 The animals are placed on an inoculation schedule to allow gradual production of immunoglobulins, most importantly immunoglobulin IgG. Sufficient antibody production usually takes up to 6 weeks. The choice of animal used to make an immune serum is more often dictated by the availability of a species, financial considerations, and tradition rather than by scientific modeling. Horses are used by the majority of antivenom producers since they are relatively easy to maintain, and large volumes of serum can be obtained at one time without harming the animals. Varying efforts are made during antivenom production to remove animal proteins such as albumin. To date, no studies have compared immune sera of different animals for human compatibility or tolerance.

The antidotal fraction of an antivenom exists as either whole IgG, or only Fab, or F(ab)2. The IgG molecule is composed of two antigen-binding fragments (Fab fragments) that are fused together and attached to two larger complement-binding fragments (Fc fragments). The larger Fc portions are generally considered to be the most antigenic, initiating most of the undesirable histamine release on infusion. Digestion of the disulfide bonds of an IgG molecule with the enzyme pepsin will cleave the Fc fragments, allowing isolation of pure F(ab)2 fragments (two fused Fab fragments). In contrast, digestion with papain cleaves the molecule more distally such that a larger Fc portion is removed from two separate Fab fragments. Both Fab and F(ab)2 molecules can be isolated with affinity chromatography, while the highly antigenic Fc portion is discarded. Although Fab and F(ab)2 are more difficult and more expensive to produce than their whole immunoglobin counterparts, they are generally regarded as less allergenic and therefore safer products.

Whole IgG antivenom has a molecular weight of 150 kDa, the largest of the three antivenom types. Because of its size, IgG is the least filterable at the glomerulus and has the smallest volume of distribution. IgG has a longer elimination half-life than either Fab or F(ab)2.26

F(ab)2 has an intermediate size (100 kDa) and elimination half-life. Preliminary pharmacokinetic studies of an F(ab)2 antivenom for scorpions demonstrate a mean time of residence within volunteers of 10 days....

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