Disclosed are methods for activating an NKT cell, methods of stimulating an immune response in a subject, methods of improving vaccine efficacy, and methods of treating an infection. Also disclosed are methods of promoting tumor rejection, treating cancer, modulating autoimmunity and inhibiting allergen-induced hypersensitivity in subjects. The methods include contacting an NKT cell with a bacterial glycolipid complexed with a CD1 molecule to activate the NKT cell. The bacterial glycolipid may be derived from a member of the Class Alphaproteobacteria.