To directly connect the recognition of protein antigens with their molecular composition, we probed antibody-antigen complexes by using native tandem mass spectrometry. 
native tandem mass spectrometry
Immunocomplexed Antigen Capture and Identification by Native Top-Down Mass Spectrometry.Antibody-antigen interactions are central to the immune response.  These results establish native top-down mass spectrometry as a viable platform for precise and thorough characterization of immunocomplexes to relate structure to function and enable the discovery of antigen proteoforms and their binding sites.
native top-down mass spectrometry