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Although the bacterial enterotoxins cholera toxin (CT) and Eschericia coli heat labile toxin (LT) are the most potent mucosal adjuvants currently known, their enterotoxicity may preclude their use in humans. Studies conducted with genetically engineered mutant molecules demonstrate that target cell binding is necessary but not sufficient for adjuvant activity and suggest that some degree of toxin activity is required for oral immunization. Although toxin activity may not be required for intranasal adjuvanticity, further studies are needed. Several studies have demonstrated that CT and LT can enhance the expression of costimulatory molecules on antigen-presenting cells and thus indirectly influence subsequent T-cell responses. Direct effects on B and T cells may also be important. Although they have not been as extensively studied as CT and LT, compounds such as liposomes, immune-stimulating complexes, microparticles and bacterial membrane proteins have also been shown to have mucosal adjuvant activity.