Snakes are not universally immune to their own venom. While it is a common belief that snakes are immune to their own venom, the reality is more complex and varies depending on the species of snake.
Some snake species have a certain level of resistance or immunity to their own venom. This resistance is attributed to specialized proteins and receptors in their blood that can neutralize or counteract the toxic effects of their venom. However for a number of venomous snakes, their own venom can be harmful or even lethal if injected into their own bodies or of the same species, which does occur. This usually happens by accidentally biting themselves.
While there are snake species with a degree of immunity to their own venom, it is not a universal characteristic among all venomous snakes. This applies to other venomous animals as well.
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