An adjuvant (from Latin, "adjuvare", meaning "to help") is a pharmacological or immunological agent that improves the immune response of a vaccine. Adjuvants may be added to a vaccine to boost the immune response to produce more antibodies and longer-lasting immunity, thus minimizing the dose of antigen needed. Adjuvants may also be used to enhance the efficacy of a vaccine by helping to modify the immune response in particular types of immune system cells: for example, by activating 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 affect the immune response in different ways, but the most commonly used adjuvants include aluminium 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 longer period of time, thus maximizing the immune response before the body clears the antigen. Examples of depot type adjuvants are oil emulsions. An adjuvant can also act as an irritant, which engages and amplifies the body's immune response. A tetanus, diphtheria, and pertussis (DPT) vaccine, for example, contains small quantities of inactivated 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 more than 70 years.
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 lymphocytes. 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 a slow release that continues to stimulate the immune system. This is now under debate, as studies have shown that surgical removal of these depots had no impact on the magnitude of IgG1 response.
As stabilizing agents
Although immunological adjuvants have traditionally been viewed as substances that aid the immune response to the antigen, adjuvants have also evolved as substances that can aid in stabilizing formulations of antigens, especially for vaccines administered for animal health.
- Analgesic adjuvants
- Inorganic compounds: alum, aluminium hydroxide, aluminium phosphate, calcium phosphate hydroxide
- Mineral oil: paraffin oil
- Bacterial products: killed bacteria Bordetella pertussis, Mycobacterium bovis, toxoids
- Nonbacterial organics: squalene
- 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 product based on peanut oil. Adjuvant 65 was tested in influenza vaccines in the 1970s, but was never released commercially.
Mechanism of immune stimulation
Adjuvants can enhance the immune response to the antigen in different ways:
- Extend the presence of antigen in the blood
- Help the antigen presenting cells absorb antigen
- Activate macrophages and lymphocytes
- Support the production of cytokines
Alum as an adjuvant
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.
An increased number of narcolepsy (a chronic sleep disorder) cases in children and adolescents was observed in Scandinavian and other European countries after vaccinations to address the H1N1 "swine flu" pandemic in 2009. Researchers found that the higher incidence correlated with the use of AS03-adjuvanted influenza vaccine (Pandemrix). However, subsequent studies by researchers at Stanford University suggested that narcolepsy could be triggered by influenza itself rather than by adjuvanted vaccines and that by avoiding vaccinations patients may actually increase their risk of developing narcolepsy. 
The adjuvant of the vaccine contained vitamin E that was no more than a day's normal dietary intake. Vitamin E increases hypocretin-specific fragments that bind to DQB1*602 in cell culture experiments, leading to the hypothesis that autoimmunity may arise in genetically susceptible individuals, but there is no clinical data to support this hypothesis.
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