The term allergy denotes a hypersensitive response within the body, to certain substances known as allergens. The allergen is not necessarily harmful in itself and most people do not respond adversely to the allergens. But people who develop hypersensitive response to the allergen can develop several symptoms, some of which can even be life threatening.
Any substance (natural or synthetic) can trigger an allergic response. During an allergic response the immune system activates the small lymphocytes known as the T cells and B cells. T cells and B cells normally produce antibodies which destroy or neutralise the foreign antigens (the allergens). But in people with allergy the allergens lead to an immune response which causes the damage of the local tissue or inflammation. Besides the T and B lymphocytes, the type of antibody known as IgE (Immunoglobulin E) is also responsible for causing allergic reaction and tissue damage or inflammation. The IgE antibodies cause excessive activation of special white blood cells known as mast cells and basophils.
The allergies can be divided into four different types (type 1 to IV) based on the damage that is caused to the tissue. The allergies can also be divided as acute and chronic allergy.
- Type I Reactions (atopic or anaphylactic reactions): When the allergen activates IgE antibody which is bound to mast cells it results in de-granulation of mast cells and production of histamine and other inflammatory agents such as leukotrienes. These substances result in allergies such as symptoms of asthma, hay fever and anaphylaxis (a severe type of allergy which can be fatal if not treated within minutes).
- Type II Reactions (cytotoxic reactions): It is caused due to the activation of IgE antibodies and these allergies are characterised by the fact that they lead to the destruction of cells within the body by antibodies.
- Type III reactions (immune-complex reactions): In this allergic reaction, the complement system is activated in response to deposition of antigen-antibody complexes on the walls of the small blood vessels. This results in the inflammatory destruction of blood vessels.
- Type IV reactions (delayed or cell-mediated immune reactions): The allergens in the type IV reaction activate the lymphocytes resulting in destruction of healthy cells. In most cases delayed or cell-mediated immune reactions occur within 12 to 24 hours after exposure to the allergen. Contact dermatitis is an example of delayed allergic reactions.
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