Allergy is an adverse immune reaction to a particle, insect bite, or physical exposure. These reactions can be reduced by using antihistamines, decongestants, combination medicines, and corticosteroids.
Antihistamines block histamine receptors, the substance released when the body is exposed to an allergen. Histamines latch on to receptors in blood vessels causing them to enlarge, creating allergic symptoms like swelling, congestion, redness, and itching. Antihistamines block the histamine receptors and prevent or weaken allergic responses.
e.g. Alerid, Claramax, Claratyne