Type I Hypersensitivity: Antibody Binding and Mast Cell Activation

In type I hypersensitivity reactions, antibodies (specifically IgE antibodies) are not bound to host cells. Instead, they are bound to high-affinity receptors (FcεRI) on the surface of mast cells and basophils. When an allergen binds to these IgE-bound receptors, it triggers the release of inflammatory mediators such as histamine, leukotrienes, and cytokines, leading to the symptoms of an allergic reaction.

In contrast, in type II and type III hypersensitivity reactions, antibodies are directly involved in damaging host cells. In type II hypersensitivity, IgG or IgM antibodies bind to antigens on the surface of host cells, activating the complement system and leading to cell lysis. In type III hypersensitivity, immune complexes (composed of antigens and antibodies) are deposited in tissues and activate the complement system, leading to inflammation and tissue damage.