What mechanism initiates Type I hypersensitivity reactions?

Type I hypersensitivity reactions, also known as immediate hypersensitivity reactions, are primarily initiated when IgE antibodies bind to specific allergens. This process involves the sensitization phase where an individual is first exposed to an allergen, leading to the production of IgE antibodies by B cells. These IgE antibodies then attach to the surface of mast cells and basophils, which are types of immune cells involved in allergic responses.

Upon subsequent exposure to the same allergen, the bound IgE antibodies recognize and attach to the allergen, triggering the mast cells and basophils to degranulate. This degranulation releases various mediators, including histamine, leukotrienes, and prostaglandins, which are responsible for the classical symptoms of allergic reactions such as vasodilation, increased vascular permeability, and bronchoconstriction.

Understanding this mechanism is crucial because it highlights how specific immune components (namely IgE and mast cells) play a central role in the pathophysiology of allergic responses. This pathway distinguishes Type I hypersensitivity from other types, such as autoimmune responses or those involving the complement system, which are not directly involved in the initial triggering of this specific hypersensitivity type.