What is Allergy
Allergy is a hypersensitive reaction of body tissues to certain substances known as allergens which in majority of people have no effect. When a person suffering from allergy is exposed to allergen which can be virtually any foreign molecule regardless if natural or synthetic the immune system activates the small lymphocytes known as the T cells and B cells. T cells and B cells are responsible for producing antibodies which destroy or neutralize the foreign antigens which are in case of allergy the allergens. However, all allergic reactions which cause damage of the local tissue or inflammation are not always caused by lymphocytes. Allergic reaction can be also caused by a class of antibody known as IgE (Immunoglobulin E) which results excessive activation of special white blood cells - mast cells and basophils.
Allergies are usually divided in four types according to the type of tissue damage they cause, while some allergic reactions are combination of several different tissue damages:
- Type I reactions (atopic or anaphylactic reactions) are caused by IgE which bounds to mast cells. When mast cells detect presence of antigen they granulate histamine and produce other inflammatory agents such as leukotrienes which dilate blood vessels and constrict air passages what results symptoms of asthma, hay fever and in severest form anaphylaxis which can be fatal if not treated within minutes.
- Type II reactions (cytotoxic reactions) are triggered by IgE and are characterized by destruction of cells within the body by antibodies.
- Type III reactions (immune-complex reactions) are characterized by activation of complement system as a reaction to antigen-antibody complexes deposited on the walls of the small blood vessels causing inflammatory destruction of blood vessels.
- Type IV reactions (delayed or cell-mediated immune reactions) are notable for interaction of antigens and lymphocytes which results destruction of healthy cells. Type IV reactions typically occur within 12 to 24 hours after exposure to the allergen. One of the most notable examples of delayed allergic reactions is contact dermatitis.