"And it is a simple thing for the Psylli to tell by the taste of the poison what kind of snake it was whose bite the healer has mastered."
The earliest accounts of immunization known as of 2016 are by the Roman poet Marcus Annaeus Lucanus (ACE 39-65) documenting the Psylli tribe in what is now Libya practicing acquired immunity to snakebite by repeated ingestion of tiny doses of the venom.
The idea of obtaining innate immunity to snakebites did not end in the classical era. There is a video on youtube from an Australian science program about aman in Wisconsin that has built up an immunity to the bite of the black mamba, one of the nastier venomous snakes. The doctor on the program explains that repeatedly injecting small amounts of the venon over a period of time build up enough antibodies over time that he build up such a reserve he has attained innate immunity to the bite.