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An adjuvant (from Latin, adiuvare: to aid) is a pharmacological and/or immunological agent that modifies the effect of other agents. Adjuvants are inorganic or organic chemicals, macromolecules or entire cells of certain killed bacteria, which enhance the immune response to an antigen. They may be included in a vaccine to enhance the recipient's immune response to the supplied antigen, thus minimizing the amount of injected foreign material. Adjuvants are also used in the production of antibodies from immunized animals. The most commonly used adjuvants include aluminum hydroxide and paraffin oil.
Immunologic adjuvants are added to vaccines to stimulate the immune system's response to the target antigen, but do not in themselves confer immunity. Adjuvants can act in various ways in presenting an antigen to the immune system. Adjuvants can act as a depot for the antigen, presenting the antigen over a long period of time, thus maximizing the immune response before the body clears the antigen. Examples of depot type adjuvants are oil emulsions. Adjuvants can also act as an irritant which causes the body to recruit and amplify its immune response. A tetanus, diphtheria, and pertussis vaccine, for example, contains minute quantities of toxins produced by each of the target bacteria, but also contains some aluminium hydroxide. Such aluminium salts are common adjuvants in vaccines sold in the United States and have been used in vaccines for over 70 years. The body's immune system develops an antitoxin to the bacteria's toxins, not to the aluminium, but would not respond enough without the help of the aluminium adjuvant.
Adjuvants are needed to improve the routing and adaptive immune responses to antigens. This reaction is mediated by two main types of lymphocytes, B and T cells. Adjuvants can apply their effects through different mechanisms. Some adjuvants, such as alum, function as delivery systems by generating depots that trap antigens at the injection site, providing slow release in order to continue the stimulation of the immune system.
Although immunological adjuvants have traditionally been viewed as substances that aid the immune response to antigen, adjuvants have also evolved as substances that can aid in stabilizing formulations of antigens, especially for vaccines administered for animal health.
Adjuvants can enhance the immune response to the antigen in different ways: