Allergies are over-reactions of the body to substances it considers to be harmful, even if they normally do not cause harm.
These over-reactions of the body that occur upon invasion by foriegn substances are defensive in nature. This is the role of our immune system.
The different ways in which the immune system defends the body are complex and sophisticated. One of its activities is to produce antibodies. Their role is to neutralise intruders, or so called antigens.
Most of the time, these immunological reactions are beneficial. But sometimes the immune system goes beyond its mandate. It over-reacts and massively defends against an offending substance that poses no real threat. These harmful reactions are called hypersensitivity reactions. Allergy is a type of hypersensitivity reaction in which the immune system produces IgE antibodies against antigens that pose no threat. The antigens that provoke such allergic over-reactions are called allergens.
There are many allergens that can trigger an allergic response. The immune system reacts in essentially the same way to any of them but the resulting allergic diseases can be quite diverse: symptoms can take a variety of forms (e.g. sneezing, coughing, itching, rash etc), can manifest themselves in different body organs (e.g. nose, skin, lungs, eyes) and with different degrees of severity.
From the very first time that our body comes into contact with a foreign substance (antigen), the immune system learns how to recognise it
and also memorises it. It then prepares a specific response for each antigen. Production of antibodies specifically designed to match and fight only a specific antigen is part of this targeted immune response.
This is why someone suffering from hay fever might have come into contact with the allergen several times without having any symptoms but once the pollen allergen has been memorised by the immune system and the body’s defences set, the person will react every time he comes into contact with that type of pollen. It takes a little longer, days or even weeks, to get a complete immune response the first time an allergen is encountered. However, thanks to the immune system’s memory, every time the same antigen appears in the body, the immune system recognises it and rapidly responds.