“The conventional wisdom is that they have circulating antibodies in their blood,” says Stephen Mackessy, Ph.D., a Professor of Biological Sciences at the University of Northern Colorado and an expert in venomous snakes. “This would protect them from their own venom, as well of venom from another snake in their own species.”
Hypothetically, if a speckled rattlesnake bit another speckled rattlesnake, the immunity they’ve built from being exposed to low levels of their own venom—kept in glands
behind their eyes and secreted when they bite—would protect them from a fatal wound. (Even though the glands are closed, they still contain blood vessels that allow the venom to circulate in their system, causing mild exposure, Mackessy says.) But a speckled rattlesnake that meets up with a cobra might have a different day entirely.
“The more distantly related the species, the more probability it would be toxic,” Mackessy says. “If a rattlesnake and cobra bit one another, without prior exposure to their venoms to build immunity, they would probably kill each other.”
Which brings us to snake handlers
They too can build up an immunity to the toxins of the snakes that they handle.
(not something that I would want to try)
And, then, we have the story of an Australian brown tree snake that bit itself and died from the bite.