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== Background ==

Interleukin-12 also known as IL-12. There are different cells including macrophages, some B cells, and neutrophils that produce IL-12 to generate immunity to pathogens. It can also stimulate the proliferation of natural killer cells, which kills cells in someone’s body that has been affected by a pathogen, and T cells. IL-12 is also a heterodimeric glycoprotein. In IL-12 there are two subunits that are di-sulfide linked. That di-sulfide bonds in not necessary for the formation of IL-12. It is also not necessary for the secretion of IL-12, but the di-sulfide bonds help to stabilize the subunits. There have been studies done to see its affects for immunotherapy

== What it is ==

IL-12 is a cytokine. A cytokine is small protein that signals the immune responses between lymphocytes and macrophages to generate cell mediated immunity, in which the body fights off pathogens without the production of antibodies.

== Structural highlights ==

IL-12 is a heterodimer, meaning it has two polypeptide chains that differ in their composition and number of amino acids. Those polypeptide chains have carbohydrates attached meaning that IL-12 is also a glycoprotein. It has two disulfide linked subunits p35 and <scene name='84/843022/P40/3'>p40</scene>. <ref name="charged">PMID:10899108</ref> The p35 subunit is similar in structure to other class 1 cytokines. The p40 subunit is similar to hematopoietic cytokines. Hematopoietic cytokines make hematopoietic cells turn into different blood cells. P35 is unstable without p40 and can not be secreted but p40 can function without p35. <ref name="charged" /> P35 has a long chain four helix bundle that binds to a soluble alpha chain receptor subunit which is p40.<ref name="charged" /> There is an <scene name='84/843022/Arg_reside/2'>arginine residue</scene> that projects from the p35 into a pocket in p40. That pocket could be a spot for an inhibitor to bind so IL-12 cannot form. When p35 and p40 combine they form IL-12 which is also known as p70. When looking at the disulfide bond between p35 and p40 they found the it is not necessary to form and secrete IL-12 but it makes sure there is stabilization between these subunits. <ref name="charged" />

== Function ==

Interleukin-12 is produced by macrophages, neutrophils, and some B cells. IL-12 helps make cytotoxic cells and natural killer cells. T cells turn into cytotoxic t cells when IL-12 is released. <ref name="charged" />  

Innate immunity is when the body has defense mechanisms that starts working within hours of a pathogen being introduced into the body. Innate is the first line of defense against pathogens so there is no specific pathogen, they attack anything. Adaptive immunity is acquired or specific. Each attack is specific to the pathogen. It is supposed to attack pathogens that are invading. If the immune system attacks the host organism an autoimmune disorder develops. Adaptive immunity is done by b or t lymphoctyes. T helper cells secrete different cytokines like IL-12 which lets IL-12 activate innate natural killer cells and adaptive cytotoxic t cells.<ref name="immunotherapy">PMID:24514955</ref> IL-12 helps mature cytotoxic t cells so they can attack specific pathogens. <ref name="immunotherapy" /> It also helps stimulate the production of natural killer cells so they can start innate immunity. IL-12 is produced by dendritic cells, and monocytes. 
 
To increase the production of IL-12 it can be done through priming or amplification. <ref name="immunotherapy" /> In priming when it binds to a ligand and goes through the toll like receptors it allows more IL-12 to pass through faster. Amplification happens through a cytokine network where different cytokines are secreted through different cells.

IL-12 has many actions. One action is stimulating the growth and cytotoxicity of natural killer cells. They also convert th0 cells into th1 cells by activating specific transcription factors. <ref name="immunotherapy" /> IL-12 also stimulated the production of <scene name='84/843022/Interferon/1'>interferon gamma</scene> which is another type of cytokine used in innate and adaptive immunity. <ref name="immunotherapy" /> Interferon gamma is secreted by t cells and natural killer cells also it promotes macrophage activation meaning it helps more macrophages to be activated so it can eat bacteria, viruses, and fungi. <ref name="interferon">PMID:10688427</ref> IL-12 also helped to introduce IgG and suppress IgE from B cells. IgG is the most common antibody released from plasma B cells and has two antigen binding sites. IgE is an antibody that is produced when your immune system reacts to an allergen. All those actions that IL-12 performs helps it with its antitumor effects. In animal studies IL-12 has been useful in tumor therapy. <ref name="immunotherapy" /> Its effectiveness is also increased when used with other therapeutic models.

IL-12 is proinflammatory meaning it promotes inflammation which contributes in developing th1 cells from th0 cells. <ref name="family">PMID:22814351</ref> It also participates in a positive feed back loop. IL-12 causes interferon gamma production from t cells. That facilitated th1 differentiation. IL-12 also induced production of interferon gamma by natural killer cells. <ref name="family" /> Interferon gamma can be a part of the positive feed back loop because transcription factors are activated. In a study done on mice, if specific transcription factors are not expressed, there can be a defect in one or both subunits of IL-12. <ref name="cytokines">PMID:19275692 </ref>

== Diseases ==

If there is a deficiency in IL-12 then cell mediated immunity becomes impaired. <ref name="cytokines" /> That makes organisms more susceptible to diseases. If interleukin-12 is over expressed it could lead to persistent inflammation which is harmful for some autoimmune disorders like multiple sclerosis. <ref name="cytokines" /> It is harmful because it makes the immune system attack more. IL-12 can be protective or detrimental depending on the infection it is aiding in attacking. It can work with other cytokines to inhibit the infectivity of HIV in macrophages. <ref name="cytokines" /> With all the various functions IL-12 partakes in it could be helpful in immunotherapy for some infections.

== IL-12 Family ==

IL-12 is apart of the interleukin-12 family. The family consist of IL-12, <scene name='84/843022/Il-23/1'>IL-23</scene>, IL-27, and IL-35. <ref name="family" />They are all heterodimeric cytokines. They are structurally similar but all have very different biological activities. IL-23 also has two disulfide linked subunits. One subunit both IL-23 and IL-12 share is p40. <ref name="cytokines" /> IL-23's other subunit acts very similary to p35 in IL-12. That subunit is  p19 and is doesn’t secrete much when not in the presence of p40. <ref name="cytokines" /> 

== References ==
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