My mother, 79, keeps having issues with infections in her prosthetic hip. she has been on a either oral antibiotics or iv antibiotics for over a year. she sees an infectious disease specialist. should she see an immunologist, as well?

I do not think your mother is going to benefit from seeing an immunologist. The foreign body, prosthesis, predisposes to increased chances of infection as well as ageing so we know she has reduced immunity. The antibiotics are just to serve as palliation since it may be more risky to subject her to another surgery to remove the prosthesis. She should keep seeing her infectious disease physician.