What is antigen escape?

What is antigen escape? This is mostly in the context of immunotherapies. We talked about how the tumor cell might lose an antigen expression and the immunotherapy is no longer effective. But it could also happen that the immunotherapy that we are using is no longer effective against that antigen in the sense that the cell has figured out a way to be resistant to that mechanism even despite the antigen still being there. So it's a question of almost loss of recognition of that antigen being a valid target anymore.