Immunotherapy holds tremendous promise for improving cancer treatment. Administering radiotherapy with immunotherapy has been shown to improve immune responses and can elicit an “abscopal effect”. Unfortunately, response rates for this strategy remain low. Herein, we report an improved cancer immunotherapy approach that utilizes antigen-capturing nanoparticles (AC-NPs). We engineered several AC-NPs formulations and by using a proteomic approach, we demonstrated that the set of protein antigens captured by each AC-NP formulation is dependent upon NP surface properties.