The simplest answer is that it contains the toxin in an inactive form, or keeps it in a cellular vesicle to protect itself. Or maybe its particular cell membrane ion pumps are simply immune. Although, generally speaking, all cells have ion pumps to alter their interior environment and local environment, this charge differential is easily noticed, and manipulated with micro-electrodes. This is the basis of the earliest work with neurons. I wanted to provide a link, but Wikipedia is a little all over the place on the topic.