An adjuvant (from Latin, adiuvare: to aid) is a pharmacological and/or immunological agent that modifies the effect of other agents. Adjuvants may be added to vaccine to modify the immune response by boosting it such as to give a higher amount of antibodies and a longer-lasting protection, thus minimizing the amount of injected foreign material. Adjuvants may also be used to enhance the efficacy of vaccine by helping to subvert the immune response to particular cells type of immune system, for example by activating the T cells instead of antibody-secreting B cells depending on the type of the vaccine. Adjuvants are also used in the production of antibodies from immunized animals. There are different classes of adjuvants that can push immune response in different directions, but 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.
Mechanisms of adjuvants
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. This is now under debate as studies showed that surgical removal of these depots had no impact on the magnitude of IgG1 response
Adjuvants as stabilizing agents
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.
Types of adjuvants
- Inorganic compounds: alum, aluminum hydroxide, aluminum phosphate, calcium phosphate hydroxide
- Mineral oil: paraffin oil
- Bacterial products: killed bacteria Bordetella pertussis, Mycobacterium bovis, toxoids
- Nonbacterial organics: squalene, thimerosal
- Delivery systems: detergents (Quil A)
- Plant saponins from Quillaja, Soybean, Polygala senega
- Cytokines: IL-1, IL-2, IL-12
- Combination: Freund's complete adjuvant, Freund's incomplete adjuvant
The mechanism of immune stimulation by adjuvants
Adjuvants can enhance the immune response to the antigen in different ways:
- extend the presence of antigen in the blood
- help absorb the antigen presenting cells antigen
- activate macrophages and lymphocytes
- support the production of cytokines
Alum as an adjuvant
Alum is the most commonly used adjuvant in human vaccination. It is found in numerous vaccines, including diphtheria-tetanus-pertussis, human papillomavirus, and hepatitis vaccines. Brain translocation of alum particles is linked to a Trojan horse mechanism, signaling the major inflammatory monocyte chemoattractant by expression of CCL2 cytokine. 
Adjuvants may make vaccines too reactogenic, which often leads to fever. This is often an expected outcome upon vaccination and is usually controlled in infants by over-the-counter medication if necessary. In addition, concerns have been raised regarding whether adjuvants may be able to trigger responses to self-proteins causing autoimmunity. Increased number of narcolepsy (a chronic sleep disorder) cases in children and adolescents was observed in Scandinavian and other European countries after vaccination in 2009 due to H1N1 “swine flu” pandemic.
Narcolepsy has previously been associated with HLA-subtype DQB1*602, which has led to the prediction that it is an autoimmune process. After a series of epidemiological investigation, the higher incidence was found to be correlated with use of AS03-adjuvanted influenza vaccine (Pandemrix). Those vaccinated with Pandemrix has almost 12 times higher risk of developing the disease. The adjuvant of the vaccine contained vitamin E that was no more than a day’s normal dietary intake. Vitamin E increases hypoceratin-specific fragments that bind to DQB1*602, leading to hypothesis that autoimmunity may arise in genetically susceptible individuals. However, no clinical data is available to support this hypothesis yet.
- Adjuvant care
- Agricultural spray adjuvant
- Combination therapy
- Crop oil
- Freund's adjuvant
- Immunologic adjuvant
- Inactivated vaccine
- Keyhole limpet hemocyanin
- Pharmaceutic adjuvant
- Reverse vaccinology
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