Hypersensitivity is a disorder of the immune system. It is defined as overreaction of the immune system to a foreign antigen. This reaction results in harming the body cells.
There are four types of hypersenstivity and some authorities include a fifth type but will discuss only four types here. Three of them are mediated by humoral immunity or by secretion of antibodies. The fourth type is cell mediated.
The mechanism of hypersensitivity in all forms involves intitial exposure to a sensitizing antigen which elicits a primary response that is called sensitization. The immune system is activated apporximately one week after the initial exposure. Any subsequent exposure to antigens following the first sensitization tiggers hypersensitivity.
Type I hypersensitivity causes the production of immunoglobulin E. This type of hypersensitivity occurs within minutes of the exposure to the antigen. The antigen attaches itself to the surface membrane of mast cells and basophils.
Subsequent exposure to the antigen causes degranulation of the mast cells and basophils. These granules contain vasoactive compounds such as histamines and serotonin which can cause vasodilatation and increased permeability of blood vessels. In addition it can cause smooth muscles contraction.
Type I hypersensitivity is characterized by the secretion of immunoglobulin E and vasoactive compounds. In addition, eosinophiles during this process secrete specific enzymes such as arylsulfatase and histaminase that can neutralize the action of histamine and leukotrienes. Thus, ameliorating in this way the effect of hypersensitivity reaction caused by this type of allergy.
Disorders that can arise from type I hypersensitivity can affect the skin causing reddening and swelling of of the skin. It can also affect the nose, causing allergic rhinitis and hay fever. It can also affect the lungs causing allergic asthma. It can also affect the intestine, causing muscles contraction and diarrhea.
Anaphylaxis is a type of hypersensitivity that is part of type I hypersensitivity. The increased secretion of vasoactive compounds causes smooth muscles contraction and vasodilatation in addition to vascular permeability. It can lead to edema and low pressure or shock. The shock can be treated with the administration of epinephrine.
Hypersensitivity type II is typified by a reaction between an antibody and an antigen on the surface of a body cell causing its killing. Some types of this hypersensitivity can cause hemolysis due to the destruction of platelets by the immune system. It can also cause a destruction of neutrophils by the immune reaction. Thus leading to susceptibility to infection.
Type III hypersensitivity occurs as a result of an interaction of an antibody with an antigen that can form an immune complex either at the site of injury or in the circulation.
Examples of diseases that are triggered by this type of hypersensitivity include poststreptococcal glomerulonephritis in which the immune complex is deposited in the basement membrane of capillaries in the glomerulus of the kidney. Another example of a disease that is triggered by an immune complex is lupus erythematosus.
The last type of hypersensitivity that we discuss here is type IV hypersensitivity. It is a delayed hypersensitivity that occurs after an exposure to the antigen in 3-4 days. Type IV hypersensitivity is mediated by cellular immunity rather than by humoral immunity.
It is mediated by T-lymphocytes that can act directly on the host cell or through the secretion of lymphokines. An example of a disease that is caused by this type of hypersensitivity is Hashimoto thyroiditis. In this disorder there is a direct T cell action against the antigen on the host cell which is the thyroid epithelial cells in this disorder. This interaction can lead to the destruction of the thyroid tissue.