Step 1: Understanding the Concept:
An allergy is an exaggerated, hypersensitive response of the immune system to common environmental substances called allergens (such as pollen, dust mites, or animal dander). When an individual is exposed to an allergen, their body mistakenly triggers a defense response mediated by IgE-type antibodies. These antibodies bind to the surface of specialized immune cells, causing them to rupture and release chemical mediators that produce common allergy symptoms like sneezing, watery eyes, and inflammation.
Step 2: Detailed Explanation:
Let's trace the cellular events during an allergic flare-up:
1. Upon encountering an allergen, IgE antibodies stimulate Mast cells (present in connective tissues) and Basophils (circulating in the blood).
2. This activation causes these cells to undergo degranulation, meaning they rapidly release stored chemical signals into the surrounding tissue.
3. The primary inflammatory chemical released is Histamine (along with serotonin).
4. Histamine acts as a local vasodilator, widening blood vessels and increasing the permeability of capillaries. This fluid shift causes redness, local swelling, itching, irritation, and smooth muscle contractions.
Among the options, neutrophils, basophils, and acidophils are cell types themselves, whereas Histamine is the specific chemical compound secreted by these cells to drive the allergic response. This matches option (A).
Step 3: Final Answer:
The substance secreted in an allergic reaction is Histamine.