The purpose of this study is to determine the safety, tolerability, efficacy, PK and pharmacodynamics of INCAGN01876 when given in combination with retifanlimab in participants with GITR expression in recurrent or metastatic HNSCC who have progressed on or after prior systemic therapy including anti-PD-(L)1 therapy. The study will consist of 2 parts: a safety lead-in part (Part 1) followed by a dose expansion part (Part 2)
