If it is the first time, the body will take some time to make specific anti-antibodies, the antivenom has plenty of time to bind to the poison.

On a future second time (specially if the first was recent) you could get a race between the kinetics of your anti-antibodies binding the antivenom and the antibodies binding the poison. I would expect it would still work, but with decreased effect.

Finally, and this is the way that it always works, the poison + antibodies (yours or external) end up making a bigger clump that is consumed by cells of the immune system (macrophages), so ultimately you always get an immune response (but that does not necessarily mean an allergic reaction)

Some patients develop a sensitivity to anti-venom. However, there is a common myth that you cannot have anti-venom more than once. It is totally false. If you do develop a sensitivity and have a reaction from a subsequent administration of anti-venom, that reaction is very easily managed in a hospital setting.

Addressing your comment below, anti-venom does not provide much if any lasting benefit. Snakebite vaccinations have not shown effective in humans or pets. First, the venom antibodies are relatively short-lived. Second, vaccinations work well when your immune system gets to have a fair foot race with an incubating infection. Snake bites don’t work that way. You can be delivered a lethal dose in half a second. No amount of vaccination can prep your body to catch up with a sudden envenomation.

It uses the same adaptive immunity that mammals (us included) use to develop antibodies for diseases. A key bit of information that seems to be missing from all the answers here is that most venoms are proteins, which is what mammalian immune systems usually produce antibodies against. (Not that we can only develop antibodies against proteins in particular - the key metric is the size of the molecule. Larger molecules have - by virtue of being large and structurally diverse - more unique structures than smaller ones, so chemical interactions between large molecules can be more specific and therefore stronger than those between small molecules.)

So armed with the knowledge that all or most mammals have similar immune systems that can develop antibodies against virtually any protein, and that venoms are proteins, it stands to reason that you can make antibodies to venom in most mammals. We use mammals like goats, rabbits, sheep, and horses to make other antibodies for scientific research, too.

Edit: A bit of a primer on poisons might be helpful here. As said above, we can develop antibodies against large poisonous biomolecules, whether they are enzymes that directly interfere with our biochemistry (making us sick), or they are receptors on viral particles that act during one step of a longer process that in the end interferes with our biochemistry (and thus make us sick). But some poisonous molecules are small, so we cannot develop antibodies against them. Arsenic (as arsenate), cyanide, heavy metals (lead, cadmium, radioactive iodine), and mustard gas are examples of such poisons. They act by either displacing small biomolecules (arsenate replaces phosphate; heavy metals replace other metal cofactors like iron, copper, and cobalt), competing for an enzyme's binding site (cyanide outcompetes oxygen), or reacting irreversibly with a biomolecule (mustard gas reacts with DNA). Because they are small, these poisons look a lot like other chemical species that occur frequently in biology (which is precisely why things like arsenic and lead are toxic), so antibodies against them would cause autoimmunity. Venom enzymes, being large, have unique structures that occur nowhere else in the target organism's own biology, and so they can be uniquely identified and bound by antibodies.

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trial and error. Guess how we figured out chocolate kills dogs. Or which mushrooms are edible.

There isn't one cancer, each of them is their own thing, made up of your own cells that mutated. So antibodies from somebody else will not work.

There are some viruses that can cause cancers and we are developing vaccines against them.

First, cancer is a a single disease. It is 100 diseases. Even breast cancer, lung cancer, or skin cancer each comprise a half dozen common types and dozens more rare types.

Second, cancer isn’t a foreign cell, like a venom protein, bacteria, or virus. Cancer is your body’s own on cells run amok, replicating without control in places they shouldn’t.

That’s incorrect. Body develops antibodies (also known as immunoglobulin) to most foreign substances (known as antigens). Virus, bacteria, fungi, allergens, toxins, venom, etc..

Venoms are mostly proteins, so yes of course they can be a target of antibodies. Antibodies recognise fairly short fragments of proteins, not an entire virus or bacteria.

Also venoms are biological, and I don't think purely chemical means anything, everything is chemical

Horses are heavy. Injecting very tiny doses of a venom makes them mildly ill for a few days, if at all. Then they can donate a gallon of blood at a time to harvest the antibodies. If you tried to get a gallon of blood out of a person, that person would die of blood loss, even after surviving the minor snake bite easily. Bonus: the horse is now safer from snake bites.

I’m sure there is something about serum compatibility or physiological similarity to humans, but I don’t know the answer.

We’re not talking about cosmetics testing here. We’re talking about saving human lives. If you want to sign up as the serum factory, more power to you. Otherwise save the indignation for the mink farms.

For the same reason we make vaccines instead of just infecting everyone with a virus. Not everyone’s immune system is strong enough.

Anti venom is an useful medical resource, but do you know which resource is even more needed? Human blood.

No sense in wasting human blood for something you can avoid, specially when a human has around 5 liters of blood total , while a horse can lose 5L and live.

https://www.zmescience.com/ecology/animals-ecology/antivenom-made-precious/

Plenty of blood mass, easy to handle and house, widely available.

It is possible for a human to immunize themselves against venom by injected low, dilute doses over time. But you risk (1) venom related toxicity, or (2) venom allergic sensitization. It's also just deeply impractical, dangerous in ways medicine does not accept for a reasonable risk to volunteers, and takes a long time to do safely (multiple small doses > one dose)

Horses are bigger, can tolerate a higher dose (due to greater muscle mass), and produce more antibodies (in terms of volume, not concentration)

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