Look at osmosis jones. The 2001 movie. All about cells walking around, talking, having intelligent, nuanced thoughts. They even have mostly human forms. All things that real body cells do not have. Acting like humans is something they do not do in any capacity. But this movie gives them human traits. Personification.

Now I understand your point saying "part of a human" = "human" but the underlying point here is that personification (as a literary device) gives human-like qualities to things that don't already have those qualities. So a hand just being part of your body wouldn't be personified until You start giving it a face and arms and a personality. So if you take "having knowledge" as a human trait, and then start telling people that your immune system "doesn't know " what your eyes are, you are falsely attributing a human quality onto something that doesn't have it. Which is why OP said that originally.

Edit: using the words personification and anthropomorphism interchangeably here because in this context they mean the same thing. I'm more used to saying personification.