when researchers test different symptoms or processes in the human body they use different cells for experimentation. how do you know how to pick the right cell line?

It's pretty obvious it many cases. For example, I'm doing work on uropathogenic E. coli, so I would obviously choose tissue cultures from the bladder or kidneys.

Yeah you basically pick the tissue that will show the most significant effect. Like if it's a disease you pick the affected tissue, the same for toxicoloy etc. There isn't really much of a question of picking an appropriate tissue in most cases. And if there is it's usually pretty limited.