anti-posion
The general term is antidote. Another term is antitoxin and in the case of venom poisonings (e.g. snake, spider, scorpion) antivenin.
Because it reverses the effects of something (anti means opposite)
Immunity
An antagonist counteracts another medication.
The scientific term pertaining to poison is "toxicology", which is the study of the nature, effects, and detection of poisons and the treatment of poisoning.
Pharmacotherapy is treatment with medication.
That would be 'toxic'
yes, the undertaker is an apporpiate term
toxemia
toxicity
long term exposure to a harmful substance
Radiation poison and cancer.