Adjuvants comprising chitosan cross-linked with, an aldehyde or mannosylated chitosan are provided herein. Methods of making the adjuvants and methods of combining or linking the adjuvants with antigens are also provided. The adjuvant-antigen combinations can be used in vaccine formulations and the vaccine formulations can be used, in methods to vaccinate animals against the source of the antigen or to enhance the immune response in a subject. A particular embodiment of the invention is a vaccine formulation comprising a microbial antigen and an adjuvant composition, the adjuvant composition comprising an open ring mannose moiety linked to chitosan to form a Schiff base, wherein the Schiff base is not reduced.
