The immune system responds to a variety of noninfectious foreign materials including pollen , drugs, bee, foods ,and chemicals. .An allergy is an inappropriate overreaction of the immune system. Common allergies are hives ,hay fever, ,eczema and asthma. Antibodies belonging to the Ig E class trigger the reaction by combing with both the antigen and the surface of mast cells, which are non circulating cells that form part of the connective tissue , to cause the release to large amounts of histamine into the blood stream. Histamines a normal initiator of the inflammatory response stimulates the dilation of arteries and increased permeability of capillaries that result in fluid build up in the interstitial areas of the affected tissues. Sneezing coughing , difficulty in breathing ,rashes ,itching are the symptoms of an inflammatory response in a regions of body.

A severe allergic reaction called as anaphylactic shock occurs when large amounts of histamines are suddenly released by the mast cells .all the peripheral arteries dilate at once causing a precipitous drop in blood pressure . it is fatal if not immediately treated.Allergies are treated with drugs that cause vasoconstriction anti histamines in the case of normal allergic reactions and epinephrine in the case of anaphylactic shock