Allergies are when the body’s immune system has a hypersensitive reaction towards innocuous substances called allergens. Common allergens include dust mites, pollen, mold, food, and animal dander, which could trigger allergic disorders such as allergic asthma, hay fever, allergic rhinitis, anaphylaxis, allergic conjunctivitis, and contact dermatitis. When a susceptible individual is newly exposed to an allergen, the immune system undergoes allergic sensitization. The antigen-presenting cell (APC) complexed with MHC II captures, processes, and displays the allergen to CD4+ T cells to initiate an immune response with Th2 cells. The Th2 cells release cytokines like IL-4 and activate B cells to produce the antibody immunoglobulin E (IgE). Secreted IgE bind to Fc receptors called FcεRI on mast cell and basophil surfaces, which completes the process of sensitization.
When the immune system encounters the same allergen, the allergen binds to IgE and causes cross-linking of IgE and FcεRI. This interaction activates mast cells or basophils to degranulate inflammatory chemical mediators such as histamine, cytokines, and leukotrienes. These mediators invoke vasodilation, mucus secretion, nerve stimulation, and smooth muscle contraction. After the early or acute allergic response, recruitment of other leukocytes (e.g. eosinophils) to the initial site of allergen challenge may induce late allergic responses.
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