What will happen if a poisonous snake bites itself?
“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.


So in the case snake were to bite itself it wouldn't suffer as much from the poison compared to the actual bite it self-inflicted.