Immunoglobulin E (IgE) is a class of antibody produced by B cells in response to allergens. It binds with high affinity to FcεRI receptors on mast cells and basophils, sensitizing them. Upon re-exposure to the allergen, cross-linking of receptor-bound IgE triggers rapid degranulation of these cells, releasing pre-formed mediators (e.g., histamine, tryptase) and synthesizing new mediators (leukotrienes, prostaglandins, cytokines).