Aluminum adjuvants are widely used in human and veterinary vaccines. They are appropriate adjuvants for vaccines that confer protection by inducing antibodies via the induction of a type 2 immune response, but they do not induce cytotoxic T cell and cell-mediated immunity. The mechanisms by which aluminum adjuvants selectively enhance the immune response are poorly understood. Following exposure to interstitial fluid in vitro and in vivo, most antigens are rapidly desorbed from aluminum adjuvants, suggesting that sustained release of antigen from a depot does not significantly contribute to the adjuvant effect of aluminum compounds. However, the adsorption of antigens onto aluminum salts may result in a high local concentration of antigen at the injection site and enhance the uptake by antigen-presenting cells. Aluminum compounds can further enhance the immune response by direct or indirect stimulation of dendritic cells, activation of complement and by inducing the release of chemokines. The relative importance of these mechanisms remains to be determined.