Which of the following is a major chemical mediator of the allergic response?

Histamine is a major chemical mediator of the allergic response and plays a crucial role in the physiological events that occur during an allergic reaction. When allergens enter the body, they trigger the release of histamine from mast cells and basophils, which are types of immune cells. Histamine binds to receptors on various cells, leading to symptoms commonly associated with allergies, such as itching, swelling, and increased mucus production. It significantly affects blood vessels, causing them to dilate and become more permeable, which contributes to the inflammation and redness that often accompany allergic reactions.

While cytokines, prostaglandins, and leukotrienes also play roles in the inflammatory response and can be involved in allergies, histamine is specifically recognized as a central mediator that initiates many of the immediate symptoms associated with allergic reactions. Cytokines are signaling proteins that mediate and regulate immunity and inflammation but are not the primary agents that cause the immediate hypersensitivity symptoms. Prostaglandins are also mediators of inflammation and pain but operate through different mechanisms. Leukotrienes are involved in prolonging the inflammatory response and are more characteristic of later stages in allergic reactions. Thus, histamine is distinct in its immediate impact during the onset of allergy symptoms.