If you have been in remission since 2015 you are most likely cured and don't need to worry about any treatment including any immunotherapy. You had Non-Hodgkin's lymphoma and so far immunotherapy has been approved for patients with Hodgkin's lymphoma.

I believe your question is whether you should be receiving immunotherapy. If you remain in complete remission almost three years after completing therapy for Stage IV large cell lymphoma you may be cured and I would not consider additional therapy until your disease recurs, if it ever does.