An allergy is an adaptive immune response, sometimes life-threatening, to an allergen Type I hypersensitivity requires sensitization of mast cells with IgE, involving an initial IgE antibody response and IgE attachment to mast cells.

On second exposure to an allergen, cross-linking of IgE molecules on mast cells triggers degranulation and release of preformed and newly formed chemical mediators of inflammation. Type I hypersensitivity may be localized and relatively minor (hives and hay fever) or system-wide and dangerous (systemic anaphylaxis )

allergy — an adaptive immune response, sometimes life-threatening, to an allergen.

Be able to identify and compare the distinguishing characteristics, mechanisms, and major examples of type I, II, III, and IV hypersensitivities; describe the differences between immediate and late-phase type I hypersensitivity reactions; list the signs and symptoms of anaphylaxis.