This document provides methods and materials related to using monovalent anti-CD3 antibodies (e.g., monovalent anti-CD3 Fab fragments) as adjuvants to increase the immune response produced against an antigen (e.g., a tumor associated antigen). For example, vaccine compositions containing monovalent anti-CD3γε Fab fragments in combination with tumor associated antigens (e.g., tumor associated antigens having little or no immunogenicity in the absence of monovalent anti-CD3γε Fab fragments) alone or in combination with adjuvants for signals two and/or three required for full activation of T cell immune function, as well as methods and materials for using monovalent anti-CD3γε Fab fragments to increase the immune response produced against an antigen (e.g., a tumor associated antigen) within a mammal (e.g., a human) are provided.
