system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5. Saw a video of a white blood cell chasing down and ingesting a bacteria, why aren't white blood cells "alive" as their own beings?
gpt
I know it's a flashback to, like, freshman biology but there are characteristics that we use to define what has life. They are responsiveness to stimuli, growth or change, ability to reproduce, metabolism and respiration, homeostasis-ness (freshman bio was a long time ago), cell structure, and reproduction. White blood cells check off a few but not all of them. 

More importantly, though, they're part of *our* cell structure. Without us, they do not exist or exibit any of those characteristics. They are alive (in the sense that they can die, so they must be alive) but it's inherently not as their own beings. They're no different than our skin cells or our teeth - they're alive... As long as they are part of us because we are alive. But they don't have life.