system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 how antivenom works.

I was reading about terrifyingly poisonous spiders and got curious.
gpt
Venom is a molecule synthesized in the body of the animal (spider in your case).  When it is injected into an animal, their body generates antibodies that are specific to that venom which flag the venom molecules for destruction by the animals immune system.  Sometimes, if the venom is too strong, they will have irreparable damage done before their body can generate antibodies.  In order to prevent death or serious injury in these cases, antibodies from another source are needed to help their immune system out/speed up the process.  To get these additional antibodies (antivenoms), scientists inject venom into an animal (sheep or horse typically) in small amounts (non-dangerous) to get that animal to create antibodies.  Then their blood is drawn and the antibodies are purified out.