Easy questions to ask–very difficult to answer succinctly and simply. Snake venom is saliva, a highly modified saliva, and is produced by modified saliva glands. 

The proteinaceous nature of snake venom was established by Napoleon Bonaparte's brother, Lucien in 1843. Proteins constitute the major portion of venom's dry weight–90% or more. Snake venom is a cocktail of hundreds, sometimes thousands, of different proteins and enzymes. 

Many of these proteins are harmless but a percentage of them are toxins. The makeup of these toxins varies widely from species to species. This complexity accounts for the widely differing effects of snakebite. 

Venoms are rich in hydrolithic enzymes, a complex mix of polypeptides, nucleases, peptidases, etc., which help digest the snake's prey. Some of them also enhance or contribute to the toxic effect of the venom. As early as 1949 it was shown that an enzyme from the Bothrops species produces a vasodilation resulting from the production of a hypotensor neuropeptide, bradykinin. This had important consequences for man leading to drugs for the control of blood pressure. 

Anti-venoms were first produced a century or more ago. Albert Calmette demonstrated that it was possible to "hyper-immunize" an animal against snakebite by graduated and increased regular dosage of that animal with the venom of that snake. He further demonstrated that a second animal could be saved after snakebite by introducing the serum of the immunized creature. This discovery is still the basis of the production of modern anti-venoms.

Milking a snake

The proteins that can kill or immobilize prey vary and differ in their effect and the percentages in which they are present in venom.

A few modern modifications have been introduced–such as the neutralization of the venom with formaldehyde before use on the animal. This removes a lot of the earlier suffering such animals endured. 

The animal of choice is the horse. Increasing doses of venom are injected until the animal becomes hyper-immunized and thereafter blood is drawn and the serum removed. The rest of the blood is transfused back into the animal. 

The serum then passes through various stages of refinement before it is released for use on humans. It contains immunoglobulins and these are digested by pepsin to isolate the antigen that neutralizes the venom. 

These antivenoms are very safe–however they are an animal protein derivative and a small percentage of people react dangerously to it. They display a hyper-allergic reaction which leads to anaphylactic shock which can kill. In a hospital situation a cocktail of anti-histamines and hydro-cortisones would be administered prophylactically. Then a small test sample of antivenom is administered and the reaction to it noted before a full dose is injected or preferably dripped into the patient in an intravenous solution. 

The production of serum from a single venom is known as a "monovalent" anti-venom and is efficacious only on the snake from which the venom comes. When a cocktail of venoms is used in the hyper-immunisation process the serum produced is a "polyvalent" serum and is effective against a range of venoms. However the addition of each venom causes a loss of efficiency and potency in the anti-venom as a whole. So a delicate balance of like venoms is usually used to produce an antivenom against the known snakes of a given area.