A half-and-half cartoon and live action film with Chris Rock as the titular character from the late-90s/early-00s. The movie was a flop, but I think it had more to do with lousy marketing. It was actually pretty good and a fun way to look at how things affect the human body.
David Hyde Pierce as the enormous Pill was a great choice, and Bill Murray was hilarious.
Around the time this came out, I, having had no idea about the movie or its premise, wrote a story that was along similar lines in creative writing. Specifically, it featured the immune system and other cells as characters and whatnot.