i think you are misunderstanding. typically random bites just get worse over time. i know they have, for me, for local centipede and scorpions. things i couldn't feel before make for local soreness for days, sometimes, now
a very carefully controlled regimen of measured dosages can detune allergic responses. that is basically all allergy shots are
self immunization should be the name. These guys inject themselves really really small amounts of venom. The immune system builds its own antibodies and make them immun.
The efficiency differs between the species. Rattlesnake immunsization does not works for a long time. If they stop injecting venom it breaks down fast. Cobravenom is another thing an the immunization works fine as far as I have read.