Meet the Taricha granulosa newt, otherwise known as the rough skinned newt.
It's a small, slow-moving creature. There's nothing apparently threatening about it. However, glands in its skin secrete a potent poison, tetrodotoxin. Tetrodotoxin, TTX for short, is so potent that just one thirtieth of the TTX found in one rough skinned newt is all it takes to kill the average human being.
Now meet Thammnophis sirtalis, otherwise known as a garter snake. It doesn't appear particularly threatening either. Even squirrels and bullfrogs prey on garter snakes. The garter snake has one claim to fame though. It's the only creature on Earth that can eat a rough skinned newt and live to tell about it.
You see, the face-off between the newt and the garter snake is more complicated than it appears on the surface. TTX is a paralytic poison which works by binding to the garter snake's nerve cell protein tsNa in order to interfere with the nerve cells' ability to allow sodium ions to move in and out of the cell. Paralysis and death can occur when nerve cells lose this ability.
The garter snake survives the newt's poison by making changes in these nerve cell proteins, changes that prevent the toxin from binding to them. While this is a rather remarkable feat, it's a dangerous one too. Nerve and muscle functioning rely on the snake's nerve cells' ability to channel sodium ions. Too much change or the wrong kind of change can be as dangerous as the newt's poison.