Usually venoms act by binding to the receptors present on the surface of the muscle cells on the victim thereby blocking the communication between the nerve cells and muscle cells in the victim.
When the toxin is already bound to the receptor, the natural neurotransmitter acetylcholine can no longer bind to the receptor as there were no free receptors left available by the toxins present in the venom. This blockage cause paralysis in the victim and in the worst case leads to death based on the nature of the venom. The receptors present on the mice and the humans are different from that of the venomous creatures like snakes. This difference prevents the venomous creatures’ venom binding to its receptors.
In snakes, sugar molecules cover the amino acid residues of the receptors thus protecting the binding of its own venom. However, the amino acid residues on the receptors are the same in the all the organisms only the clouding of the residues by sugar molecules make the venomous organisms resistant to their own venom.
Researchers have found that only two groups of animals, snakes and mongooses have sugar molecules on their receptors. The different types of venom attack different tissues in different ways, so a species of snake can ever become completely immune to the venoms of every other species of venomous snakes found in nature. Snakes are immune to the venom that most species of their own species. For example, in an attempt to stimulate or resist copulation, snake species bite one another during sexual combat. Snakes engage in sexual combat display immunity to the venom of their own species which is a must for their survival.
Other mechanisms that are protective for the snakes are as follows: Venom glands of the snakes make venom and specialized cells lining the venom gland protect the venom getting into their blood stream. As long as the venom does not get into the blood stream, it is safe for the animal.