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Type I reaction is called IMMEDIATE hypersensitivity reaction. This is also known as allergy. This reaction is triggered by an allergen, and this is acquired, predictable and rapid. White blood cells called mast cells and basophils are commonly involved.
Type II reaction is Antibody-dependent cell-mediated cytotoxicity. A foreign cell is, which has foreign membrane surface antigens, are attacked by cells that produce antibodies that are bound to the target cell and kills it. Natural killer cells, macrophages, neutrophils and eosinophils are also important in this reaction.
Type III reaction is the Immune Complex response, which occurs when the antigen-antibody complex presented by Type II is not enough to fend off the invading microorganism. Small immune complexes are formed, but are not cleared from the body. These complexes deposit in tissues, where they can cause damage. One good example of this is SLE.
Type IV reaction is delayed-type hypersensitivity, which takes 2 to 3 days to develop. It is cell-mediated, meaning that killer cells such as CD4+ helper T-cells release enzymes and cytokines which directly destroy the invader, unlike Type II which depends on the production of antibodies.
Hope this helps.
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