Explanation:
Henrietta's cells are considered immortal because they don't die easily, and they just keep multiplying. They multiply until they fill the container they are in, and they don't need very specific conditions to thrive. Regular human cells die quickly, but hers lasted, and still last to this day. The sample stolen from her contained cancer cells, which multiply exponentially unless stopped by an outside force. They are, in a way, 'immortal'. Since the cells stolen from her were also cancerous, they are also considered 'immortal'.
