Editing Allergy (section)

===Acute response===
[[File:Allergy degranulation processes 01.svg|thumb|Degranulation process in allergy. Second exposure to allergen. '''1''' – antigen; '''2''' – IgE antibody; '''3''' – FcεRI receptor; '''4''' – preformed mediators (histamine, proteases, chemokines, heparin); '''5''' – [[granule (cell biology)|granules]]; '''6''' – [[mast cell]]; '''7''' – newly formed mediators (prostaglandins, leukotrienes, thromboxanes, [[Platelet-activating factor|PAF]]).]]
In the initial stages of allergy, a type I hypersensitivity reaction against an allergen encountered for the first time and presented by a professional [[antigen-presenting cell]] causes a response in a type of immune cell called a [[T helper cell|T<sub>H</sub>2 lymphocyte]], a subset of [[T cell]]s that produce a [[cytokine]] called [[interleukin-4]] (IL-4). These T<sub>H</sub>2 cells interact with other [[lymphocytes]] called [[B cell]]s, whose role is production of antibodies. Coupled with signals provided by IL-4, this interaction stimulates the B cell to begin production of a large amount of a particular type of antibody known as IgE. Secreted IgE circulates in the blood and binds to an IgE-specific receptor (a kind of [[Fc receptor]] called [[FcεRI]]) on the surface of other kinds of immune cells called [[mast cell]]s and [[basophil]]s, which are both involved in the acute inflammatory response. The IgE-coated cells, at this stage, are sensitized to the allergen.<ref name=Janeway/>

If later exposure to the same allergen occurs, the allergen can bind to the IgE molecules held on the surface of the mast cells or basophils. Cross-linking of the IgE and Fc receptors occurs when more than one IgE-receptor complex interacts with the same allergenic molecule and activates the sensitized cell. Activated mast cells and basophils undergo a process called [[degranulation]], during which they release [[histamine]] and other inflammatory chemical mediators ([[cytokine]]s, [[interleukin]]s, [[leukotriene]]s, and [[prostaglandin]]s) from their [[granule (cell biology)|granules]] into the surrounding tissue causing several systemic effects, such as [[vasodilation]], [[mucus|mucous]] secretion, [[nerve]] stimulation, and [[smooth muscle]] contraction.

This results in [[rhinorrhea]], itchiness, dyspnea, and anaphylaxis. Depending on the individual, allergen, and mode of introduction, the symptoms can be system-wide (classical anaphylaxis) or localized to specific body systems. Asthma is localized to the respiratory system and eczema is localized to the [[dermis]].<ref name="Janeway" />