English to German Medical Medical (general)

Hello,

I'm not sure what the first line means in this sense. The content is about patients with rare diseases and their problems with finding a diagnosis and the whole care process in connection with doctors. About healthcare providers who refer to different specialists or centers and so on. Now I found this sentence, and I don't know how to understand "first line."

(“expertise centre refers to first line” and “collaboration between expertise centre and first line - advisory function”


It would be lovely if somebody could help me with it.

Thank you so much!