The body's own immune system protects us from infectious organisms and harmful chemicals. If foreign substances (antigens) come into contact with the skin or the lining of the nose, mouth, eyes or gastrointestinal tract, an alarm may be set off in the immune system and a series of defensive reactions unfold. An important step in this process is the formation of antibodies by the immune system. These are proteins which recognise the antigens and bind tightly onto them making it easier for the body to eliminate them.
Allergies occur when a person's immune system becomes unusually sensitive to a certain substance such as pollen or dust. These substances are then termed allergens. The antibodies (frequently from the IgE class) that are produced to attack the allergen also lead to a release of various substances including histamine. Histamine is released from a particular kind of cell called a mast cell and is responsible for causing most of the allergy symptoms. So, it is not the allergens that cause allergy symptoms but the way in which the body's immune system reacts inappropriately to them. Most people do not produce an excessive reaction when exposed to pollen and dust, but some people's bodies overreact by producing lots of histamine and other substances that create the common allergy symptoms of runny nose, sneezing, itching eyes and nose and in some people wheezing.