Adjuvant properties of aluminum and calcium compounds

It is likely that aluminum compounds will continue to be used with human vaccines for many years as a result of their excellent track record of safety and adjuvanticity with a variety of antigens. For infections that can be prevented by induction of serum antibodies, aluminum adjuvants formulated under optimal conditions are the adjuvants of choice. It is important to select carefully the type of aluminum adjuvant and optimize the conditions of adsorption for every antigen since the degree of adsorption of antigens onto aluminum adjuvants markedly affects immunogenicity. The mechanism of adjuvanticity of aluminum compounds includes formation of a depot at the site of injection from which antigen is released slowly; stimulation of immune-competent cells of the body through activation of complement, induction of eosinophilia, and activation of macrophages; and efficient uptake of aluminum-adsorbed antigen particles by antigen-presenting cells because of their particulate nature and optimal size (< 10 microns). Limitations of aluminum adjuvants include local reactions, production of IgE antibodies, ineffectiveness for some antigens, and inability to elicit cell-mediated immune responses especially cytotoxic T-cell responses. Calcium phosphate, which has adjuvant properties similar to aluminum adjuvants, has the potential advantages of being a natural component of the body and of not increasing IgE production. There is a need for alternative adjuvants, particularly for diseases in which cell-mediated immune responses are important for prevention or cure.