Objective: To determine the Overall Response Rate (ORR) to Imprime PGG + pembrolizumab in subjects with advanced Squamous Cell Carcinoma of the Head and Neck (SCCHN) Safety: To characterize the safety of Imprime PGG + pembrolizumab given in combination Hypothesis: Restore (for subjects who have failed pembrolizumab mono therapy) or enhance (for subjects who actively experiencing SD) sensitivity to checkpoint inhibitors (CPI) by appropriate and effective stimulation of the subject's innate and adaptive immune systems by combining Imprime PGG with pembrolizumab.