Final answer:
(A) Histamine is the main chemical mediator of allergic reactions. Antihistamines are used to alleviate mild allergy symptoms by blocking the action of histamine, while epinephrine is used to treat severe reactions like anaphylactic shock.
Step-by-step explanation:
The principle chemical mediator of allergic reactions is histamine. During an allergic reaction, allergens cause IgE-primed mast cells to release histamine into the body, which leads to various allergic symptoms like itching and swelling. To counteract these effects, antihistamines can be administered because they prevent histamines from causing these symptoms.
In cases of severe allergic reactions, like anaphylactic shock, epinephrine is used as it quickly counteracts the massive release of histamines that can be life-threatening. Antihistamines and epinephrine are different types of drugs; antihistamines are mainly used to reduce or eliminate mild allergy symptoms, while epinephrine is used in emergency treatment for anaphylaxis.