An adjuvant (from Latin, adjuvare: to aid) is a pharmacological 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 a vaccine by helping to modify the immune response to particular types of immune system cells; for example, by activating the T cells instead of antibody-secreting B cells depending on the purpose 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 provide immunity themselves. 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 engages and amplifies the body's 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 routing and adaptive immune responses to antigens. This reaction is mediated by two main types of lymphocytes, B and T cells. Adjuvants 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 that continues to stimulate 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 (See Quillaia), Soybean, Polygala senega
- Cytokines: IL-1, IL-2, IL-12
- Combination: Freund's complete adjuvant, Freund's incomplete adjuvant
- Food Based oil: Adjuvant 65, which is a peanut oil based product. Adjuvant 65 was tested in influenza vaccines in the 1970s, but was never released commercially (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2130368/).
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 investigations, researchers found that the higher incidence correlated with the use of AS03-adjuvanted influenza vaccine (Pandemrix). Those vaccinated with Pandemrix have almost a 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
- "ABC News: Swine Flu Vaccine: What The Heck Is an Adjuvant, Anyway? (2009)". Abcnews.go.com. 2009-08-11. Retrieved 2010-06-14.
- "Definition of immunological adjuvant -- NCI Dictionary of Cancer Terms". www.cancer.gov. Retrieved 2010-08-27.
- "Adjuvants as stabilizing agents". Benchmark Biolabs, Inc. Retrieved 2013-05-19.
- "Boostrix Prescribing Information" (pdf). GlaxoSmithKline. 2009. Retrieved 2013-05-19.
- Clapp, Tanya; Siebert, Paul; Chen, Dexiang; Jones Braun, Latoya (2011). "Vaccines with aluminium-containing adjuvants: Optimizing vaccine efficacy and thermal stability". Journal of Pharmaceutical Sciences 100 (2): 388–401. doi:10.1002/jps.22284. PMC 3201794. PMID 20740674.
- Leroux-Roels G (31 August 2010). "Unmet needs in modern vaccinology adjuvants to improve the immune response". Vaccine 28 (S3): C25–3. doi:10.1016/j.vaccine.2010.07.021. PMID 20713254.
- Hutchison S, Benson RA, Gibson VB, Pollock AH, Garside P, Brewer JM (March 2012). "Antigen depot is not required for alum adjuvanticity". FASEB J 26: 1272–1279. doi:10.1096/fj.11-184556. PMC 3289510. PMID 22106367.
- Marrack, Philippa; Amy S. McKee; Michael W. Munks (2009). "Towards an understanding of the adjuvant action of aluminium". Nature Reviews Immunology 9 (4): 287–293. doi:10.1038/nri2510. ISSN 1474-1733.
- Gherardi RK, Eidi H, Crépeaux G, Authier FJ, Cadusseau J (5 February 2015). "Biopersistence and brain translocation of aluminum adjuvants of vaccines". Frontiers in Neurology 6: 4. doi:10.3389/fneur.2015.00004. PMC 4318414. PMID 25699008.
- Masoudi, Sanita; Daniela Ploen; Katharina Kunz (23 May 2014). "The adjuvant component α-tocopherol triggers via modulation of Nrf2 the expression and turnover of hypocretin in vitro and its implication to the development of narcolepsy". Vaccine 32 (5): 2980–2988. doi:10.1016/j.vaccine.2014.03.085. ISSN 1474-1733. PMID 24721530.