Final answer:
Allergic individuals produce allergen-specific IgE antibodies that bind to mast cells. Upon re-exposure to the allergen, these antibodies cross-link and trigger mast cell degranulation, releasing inflammatory molecules that cause allergy symptoms.
Step-by-step explanation:
Allergic reactions are initiated by the immune system's response to what it perceives as harmful substances, known as allergens. In susceptible individuals, allergen-specific IgE antibodies are produced during a first exposure to an allergen. These IgE antibodies bind to mast cells through their Fc receptor. Upon subsequent exposures to the same allergen, these antibodies cross-link due to the presence of the allergen, which induces the mast cells to degranulate. Degranulation caused by the cross-linking of IgE releases a variety of inflammatory molecules, including histamine, leukotrienes, and cytokines, which lead to the clinical manifestations of an allergic reaction. The symptoms of these reactions can range from mild, such as sneezing or itchy eyes, to severe, potentially life-threatening conditions like anaphylactic shock. The primed mast cells are now ready to release these mediators immediately upon second exposure to the allergen, leading to a fast allergic response.