Some people are highly sensitive to one or more substances. 

These substances are called ALLERGENS and they are generally weak antigens. Antibodies to allergens combine with receptors on the body’s mast cells. Every person is not allergic to all allergens nor does the first exposure to an allergen causes Allergy. It only helps to form antibodies. However, on second exposure the allergens combine with anti body - bound mast cells.

This complexing causes the cells to burst and release histamines which are responsible for inflammatory responses. Allergic reactions are usually very rapid. The most common manifestations are  limitation of the mucous membrane, frequent sneezing, skin rashes, watering of eyes and inability to breathe.

When large amount of histamine is suddenly released by the mast cells, a severe allergic reaction occurs, that is known as Anaphylactic shock. For example some persons die even to a single dose of penicillin.