COPR Advanced Care Paramedic (ACP) Practice Exam

Allergic responses are driven by mast cells in tissues and basophils in the blood. These cells carry high-affinity IgE receptors (FcεRI). When an allergen enters and binds to IgE on these receptors, it cross-links the IgE and triggers rapid degranulation. The immediate release includes histamine, proteases, and other mediators that cause the classic symptoms of an allergic reaction—vasodilation, increased vascular permeability, smooth muscle constriction, and mucus production. This is the initiating phase that starts the cascade.

In the later phase of the reaction, eosinophils are recruited and contribute to ongoing inflammation and tissue damage, but they are not the primary initiators of the allergic response. Neutrophils, meanwhile, are more associated with bacterial infections and general acute inflammation rather than the hallmark initiating allergy mediators.