Adjuvants can enhance the immune effect of antigens in vaccines. Most vaccines are supplemented with adjuvants. At present, there are several kinds of adjuvants approved for human body: mineral salts such as aluminum adjuvant and calcium phosphate adjuvant; emulsions such as MF59, AS03 and AF03; immune stimulants such as TLR9 stimulant CpG. Because different adjuvants have different properties and applicable antigens, the effect of existing adjuvants on some vaccines still under development is not ideal. The organism needs to rely on the mucosa to form the first barrier to prevent pathogens from invading, and at the same time rely on cellular immunity to remove intracellular pathogens and kill pathological cells. Traditional adjuvants often fail to enhance mucosal immunity and cellular immunity. The development of vaccines and some immunotherapies still requires the development and application of some new adjuvants.

Initially, it was believed that the mechanism of immune adjuvants that could enhance the specific immune response of the body to antigens was the "warehouse" effect, i.e. during the immune response to vaccines, adjuvants played the role of storing antigens, and then slowly released antigens to stimulate the body's immune system for a long time. Recent studies have found that immune adjuvants can also affect antigen-presenting dendritic cells, thereby enhancing immune response and effective memory immune response. In the body, dendritic cells act as "sentinels". By monitoring the immune environment of the body, dendritic cells can identify necrotic cells, cytokines, bacteria and viruses released after injury. These dangerous signals can activate dendritic cells, thereby promoting the first immune response of lymphocytes with corresponding receptors migrating from lymph nodes through lymphatic vessels. There are some immune adjuvants whose ability to simulate dangerous signals is directly related to their effects. Because these dangerous signals are accompanied by damage to the body, they can cause inflammation at the injection site or in the whole organism. There are also adjuvants that can induce organisms to secrete different kinds of cytokines, thereby controlling or altering the type of immune response of organisms to antigens.

Aluminum salt adjuvant is a kind of widely used adjuvant. Aluminum hydroxide gelatin, alum, tricalcium phosphate and so on are commonly used. Aluminum salts adjuvant is a typical short-term storage, which can wrap antigen in the injection site, and is not easily lost due to liver clearance. Aluminum salts adjuvant can also activate complements, promote macrophages to secrete pro-inflammatory factors IL-1beta and IL-18, and enhance Th2 immune response induced by DNA vaccine test, and make the immunogenicity of DNA vaccine change from Th1 response to Th2 response. Oil emulsion adjuvant is a kind of adjuvant which mixes oil substances and emulsifiers in a certain proportion. Traditional formulations are divided into two types: oil-in-water emulsion and oil-in-water emulsion. The most famous water-in-oil emulsion is Freund's adjuvant, which consists of mineral oil, emulsifier and killed mycobacterium. If all three ingredients are in adjuvant, the adjuvant is called Freund's Complete Adjuvant, and the adjuvant lacking Mycobacterium is called Freund's Incomplete Adjuvant. Freund's complete adjuvant is a very powerful adjuvant, which can stimulate the body to produce strong humoral and cellular immunity, but it is more toxic. FIA is less toxic than FCA, but less active than FCA. FIA is a typical adjuvant that only induces Th2 cytokine-induced antibodies. Both adjuvants have poor stability and are difficult to preserve for a long time. Some ingredients extracted from natural plants have adjuvant activity, because they have the advantages of natural sources, low toxicity, easy metabolism and not easy to produce drug resistance, which has attracted more and more attention in adjuvant research in recent years. Natural immune adjuvants include polysaccharides, flavonoids, saponins, propolis, etc.

Ideal Autoimmune Disease Vaccine adjuvants should have some characteristics: small side effects on specific animals; stable and lasting effects; low production costs; appropriate immune response to induce cellular or humoral immune intensity to achieve protective effect; the possible side effects of different administration routes have been clearly studied. In the future, the research directions of adjuvants should be as follows: firstly, to study the mechanism of cellular and molecular immunity of known adjuvants from both natural and acquired immunity; secondly, to find new adjuvants that can improve cellular and humoral immunity, cytokine secretion and immune memory, we may call it Adjuvant Optimization development.

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