Using relevant example explain how allergic reactions take place in human beings.

-An allergic reaction/ allergy is a hypersensitive response of the body immune system to an antigen; e.g. Asthma/Hay fever; It is brought about when the body reacts by overproducing antibodies against harmless antigens; Allergic people are hypersensitive to materials like dust/ pollen grains/ some foods/ some drinks/some drugs ( e.g. penicillin, chloroquine and asprin)/ some pollutants/ fungal spores/ feathers/ fur/ strong perfumes; The antigen-antibody reaction occurs on the surface of the body cells; which burst open and release histamine. Histamine increases the permeability of epithelial cells of capillaries; making the intercellular spaces become filled with tissues fluid and the tissue swells up; Histamines cause inflammation/ itching/ swelling/ pain/ breathing difficulties/ constricting of bronchi/ dilation of capillaries/ excessive secretion of mucus/ diarrhoea/ vomiting/ sneezing/ coughing/ wheezing;