An adjuvant is any substance that enhances the response of the immune system to the foreign material termed an antigen. The particular antigen is also referred to as an immunogen. An adjuvant can also be any substance that enhances the effect of a drug on the body.

When antigen is injected into an organism being used to raise antibodies the effect is to stimulate a greater and more prolonged production of antibody than would otherwise occur if the antigen were injected alone. Indeed, adjuvants are very useful if a substance itself is not strongly recognized by the immune system. An example of such a weak immunogen is the capsule exopolysaccharide of a variety of