Adjuvants

Abstract

The most widely recognized and utilized adjuvants are the water-in-oil emulsions initially developed by Freund. These emulsions are generated by mixing an aqueous solution of soluble antigen with an equal volume of oil through a process known as emulsification. During this procedure the antigen-containing water droplets become entrapped in the oil, forming particulates in a very viscous emulsion. For adjuvant activity, it is critical that these thick emulsions remain stable and that the components do not separate after mixing. Upon administration into the host’s tissue, this viscous mixture acts as a depot of antigen. The potency of water-in-oil emulsions can be increased by incorporating immunostimulants. The classic example is complete Freund’s adjuvant (CFA), which consists of the aqueous antigen solution, mineral oil, an emulsifying agent, and heat-killed Mycobacterium tuberculosis, a very potent immunomodifier. Incomplete Freund’s adjuvant (IFA) is identical to CFA except that the M. tuberculosis is omitted. Freund’s adjuvants have been used and continue to be used extensively with antigens to stimulate the production of high concentrations of good quality antibodies. There are, however, complicating factors that need to be considered. The emulsions are rather tedious to generate and can contribute to the degradation of protein antigens. Freund’s adjuvants are known to be inherently toxic, frequently resulting in granulomas, sterile abscesses, and ulcerations following administration. To reduce the extent of these complications, most protocols recommend using CFA for the primary immunization only and then using IFA for all subsequent vaccinations. Furthermore, due to increasing concerns for research animals, many animal care facilities have banned or restricted the use of CFA, while encouraging the use of other adjuvants.