What substance can cause severe reactions in individuals sensitive to insect stings?

Venom is the correct answer because it is the specific biological substance that is injected into the body by insect stings, such as those from bees, wasps, and hornets. Venom contains various proteins and enzymes that can trigger severe allergic reactions in some individuals. These reactions can range from localized swelling and pain to systemic anaphylaxis, which is life-threatening. Individuals who are sensitive or allergic to insect venom may experience these severe reactions due to their immune system responding violently to the components within the venom.

While hormones, saliva, and digestive enzymes may have their roles in various biological processes, they do not specifically relate to the severe reactions associated with insect stings. Hormones are involved in signaling within the body, saliva aids in digestion and oral health, and digestive enzymes are used to break down food. None of these substances are responsible for the immediate and severe immune responses caused by venom. This is why venom is specifically recognized as the harmful agent that can provoke severe reactions in those who are sensitive to such stings.