Horse serum is made by taking horse blood that has developed immunities to toxins. The clotted blood was separated using chemical techniques.

Scarlet fever

RhoGAM

treated with antibiotics and immune serum. Antibiotics are used at the first sign of infection. Immune serum is a source of antibodies that help fight infections. Interferon gamma is an experimental treatment for CGD that has shown promising results

Simply because a horse is a large animal, who's immune system can cope with being injected with small doses of venom. The amount injected is no-where near enough to cause the animal to suffer - but enough to 'trick' the horse's immune system to produce anti-bodies to fight the toxins.

The horse's blood is 'harvested' after allowing the immune system sufficient time to produce large quantities of anti-bodies. The fluid is then spun in a centrifuge to separate out the different blood components. The anti-bodies are kept - and 'grown' in a laboratory to make large quantities of anti-venin..