Antihistamine drug crystals, coloured scanning electron micrograph SEM. Antihistamines block the effect of the chemical histamine, a natural chemical produced by the immune disease_fighting system of the body in response to an allergen such as pollen. Histamine causes symptoms such as itching, swelling, or reddening. Antihistamines are also used in cough remedies, as sedatives and as an emergency treatment for anaphylactic shock a severe allergic reaction.