At the beginning of Riddick the eponymous protagonist grabs one of the smaller mud-crawling beasts in order to collect some of its venom. He then constructs his own home-made injector and injects it into the dog he holds caged, only to see it fall asleep and awaken later on. After that he repeatedly injects the venom into himself. But why does he do this?
First I thought he might want to immunize himself against the venom, yet in this case I wonder why this idea wasn't picked up in the further course of the movie, given the heavy attention this whole venom draining and injecting process received at the beginning.
Furthermore, near the end of the movie Vargas gets bitten by one of the smaller beasts and immediately dies from the venom, only to have Riddick say "the smallest are the worst, they're full of venom". So what should I make out of all those elaborations at the beginning?