Adjuvants

'Adjuvant' is derived from the Latin adjuvare-to help. It is a substance added to a vaccine to stimulate the immune response to the active component of the vaccine (termed the antigen or allergen), which on its own could sometimes have only a slight effect. Adjuvants are often (but not always) relatively insoluble molecules combined with the antigen, providing a depot effect via slow dissolution after injection. This overcomes the problem of rapid loss of free antigen from the injection site. Adjuvants may also stimulate cells involved in the immune response.

A further advantage of adjuvants in allergy vaccines is that the depot effect provides a safety bonus, slowly releasing the allergen or allergoid such that the patient receives a given dose far more gently over a longer time period.
Adjuvants used in allergy vaccines are usually either 'alum' (a general term for various aluminium salts) or the natural amino acid L-tyrosine. Some newer vaccines may carry a further adjuvant which will specifically assist the immune system; for example, monophosphoryl lipid A (MPL®).