946x946px In<unk> , B cells are stimulated (by CD4+ Th2 cells) to produce IgE antibodies specific to an antigen. The difference between a normal infectious immune response and a<unk>  response is that in<unk> , the antibody is IgE instead of IgA, IgG, or IgM. During sensitization, the IgE antibodies bind to FcεRI receptors on the surface of tissue mast cells and blood basophils. Mast cells and basophils coated by IgE antibodies are "sensitized". Later exposure to the same allergen cross-links the bound IgE on sensitized cells, resulting in anaphylactic degranulation, which is the immediate and explosive release of pharmacologically active pre-formed mediators from storage granules and concurrent synthesis of inflammatory lipid mediators from arachidonic acid; some of these mediators include histamine, leukotriene (LTC4 and LTD4 and LTB4), and prostaglandin, which act on proteins (e.g., G-protein coupled receptors) located on surrounding tissues. Figure 1: Mediator release from mast cells Figure 2: Model of genesis of mast cell secretory granules Figure 3: Lipid body biogenesis Table 2: Stimuli-selective mediator release from mast cells The principal effects of these products are vasodilation and smooth-muscle contraction.<unk>  can be further classified into immediate and late-phase reactions. Within minutes of exposure to an antigen, the immediate hypersensitivity occurs, releasing histamines and lipid mediators which are responsible for the initial allergic reaction response. However, about 4-12 hours after antigen exposure, a cough and wheezing may persist in the patient, along with swelling and redness of the skin. This is known as the late-phase hypersensitivity reaction which can last from approximately 1-3 days and is caused by the release of additional mediators from the mast cells and basophils. +List of a few mediators released by mast cells in<unk>  and their actions Vasodilation and increased permeability *Histamine *PAF *Leukotriene C4, D4, and E4 *Prostaglandin D2 *Neutral proteases Smooth muscle spasm *Histamine *PAF *Leukotriene C4, D4, and E4 *Prostaglandin Leukocyte extravasation *Cytokines (e.g. chemokines and TNF) *Leukotriene B4 *Chemotactic factors for neutrophils and eosinophils Unless otherwise specified, the reference for this table is: The reaction may be either local or systemic. Symptoms vary from mild irritation to sudden death from anaphylactic shock.
Type I hypersensitivity
Type II hypersensitivity
Type III hypersensitivity
Type IV hypersensitivity
Histamine intolerance
Hypersensitivity
Pseudoallergy
Allergy