What is an Adjuvant?
An adjuvant is a substance that is co-injected with antigen in order to help stimulate the adaptive immune system into producing antibodies against the antigen.
Purpose of Adjuvants
Antibodies are produced by injecting antigens into an animal host to stimulate the production of antibodies that are directed against the antigens. However, injecting antigen alone may not elicit a desired immune response, since the immune system will rapidly remove antigen. Immunizing with only antigen prevents a strong immune response of high antibody titers, and shortening the time required for affinity maturation of high antibody affinity.
To prevent this undesired immune response, adjuvant is mixed with antigen before immunization. This process delays immediate antigen removal from the immune system, making adjuvants a necessary part of antibody development.
The Four Ways Adjuvants Work
Activate Antigen Presenting Cells to Show T Cells That Foreign Particles Are Present
Adjuvants increase recruitment and activation of antigen presenting cells (APCs). APCS are immune cells that engulf foreign particles, digest them into small fragments, and then present the fragments to T cells. Once activated T cells, activate the B cells that produce antibodies.
Inducing Release of Cytokines that Activate T Cells
Adjuvants can indirectly activate T cells by releasing complexes called phagosomes, which bind to T cells. The T cells then release cytokines that activate B cells to produce antibodies. This effect enhances the degree of antibody production against the foreign antigen that has entered the animal.
Targeting Antigens to Specific Locations
Adjuvants can induce an immune reaction to antigens at specific locations in an organism where the adjuvant was injected. Adjuvants activate the innate immune locally, which draws T cells that are circulating in the blood stream to that location.
Slow-Release of the Antigen
Adjuvants can control the rate at which antigens are released into the blood stream. This is called the depot effect. The adjuvant and antigen are trapped in a polymer, which slows the rate at which the chemicals and antigens leach into the surrounding tissue and into the circulatory system.
6 Types of Adjuvants
Mineral Salts
Aluminum salt is a common adjuvant. It is good at inducing a Th2 immune response, but is less effective for inducing a Th1 response. The Th2 response results in B cells producing antibodies that neutralize the antigen. The Th1 response results in B cells that produce antibodies that opsonize, or cling to, antigens so that other immune cells can recognize and kill things that are coated with antibodies.
Oil Emulsions
Mixtures of oil and water induce strong immune reactions. They are good at inducing a Th2 immune response. They are also good for creating the slow-release effect of antigen depots.
Microbial Products
Sugars from the cell wall of microbes are foreign particles to animals. These polysaccharide chains can induce a severe immune reaction, due to the evolutionary arms race that has been going on between microbes and animals.
Saponins
Saponins are steroid molecules that have sugar chains attached to them. They naturally occur in plants and some microbes. Their advantage is that a low dose can trigger a intense immune response.
Synthetic Products
These molecules bind to and activate the PRR and TLR receptors in immune cells. These receptors send signals to the nucleus that cause the activation of genes that tell the cell to sound the alarm of infection to its neighbors.
Cytokines
Interferons (IFN) and interleukins (IL) are naturally occurring chemicals that are released by immune cells in order to activate each other. Specific types of these molecules can elicit distinct responses in immune cells.
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