Once a patient has been exposed to an antigen (i.e. peanuts, bee venom) the antigen is presented to CD4+ helper cells which become TH2 cells. These TH2 secrete IL-4 which promotes B cell production of IgE antibody that goes on to bind to mast cells.
This reaction takes minutes to occur. There is also a late phase reaction that occurs due to recruitment of inflammatory cells.
Upon antigen reexposure, the IgE already bound to mast cells cross-links and activates signal transduction pathways.
Activated signal transduction pathways in presensitized mast cells (and basophils) lead to mast cell degranulation.
Mast cell degranulation leads to release of vasoactive amines, particularly histamine.
Histamine release results in vasodilation and rubor (redness), calor (heat) and tumor (swelling).
This is a systemic reaction that affects many organ systems and can result in distributive (vasodilatory) shock, shortness of breath and death.
Local allergic reactions are Immediate (Type 1) Hypersensitivity reactions. These are the type of reactions you experience to pollen and various animals.
An Epi Pen must be administered to a patient that experiences systemic anaphylaxis immediately to prevent sequelae such as laryngeal edema, which can lead to airway blockage and death.
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