Question

Explain in detail type I and III hypersensitivity

Answer 1

Hypersensitivity is best described as the exaggerated or abnormal reaction of the immune system to a foriegn substance. Sometimes hypersensitivity also occurs in response to the body's own cells. Hypersensitivity is of four major types which are as follows

Type I hypersensitivity or allergen mediated

Type II hypersensitivity or antibody mediated cytotoxicity

Type III Hypersensitivity or immune complex mediated

Type IV Hypersensitivity or delayed hypersensitivity.

Type I hypersensitivity or allergen mediated hypersensitivity :

The name itself indicates that this type of hypersensitivity is triggered by the exposure of common allergens such as dust or pollen which usually do not cause major problems. When the body is exposed to these allergens, they are presented to the mast cells for the proper antibody production. At times these allergens cross link to the surface antibodies IgE resulting in the release of mast cell granules known as histamines. The histamine molecules reach the site of infection and initiates allergic response by dilating the blood vessels. The dilated blood vessels result in increasing blood flow leading to swelling and inflammation in the area of infection. This response to the allergen occurs almost instantly and the symptoms are evident within minutes of infection. For this type of exaggerated response the mast cells must be pre-sensitized, meaning that the mast cells have already fought with the same allergen before.

Type I hypersensitivity majorly leads to various allergies out of which seasonal allergies are most common. The symptoms may include sneezing, coughing, nasal congestion, eczema etc.

Type III Hypersensitivity or immune complex mediated hypersensitivity

Type III hypersensitivity often leads to tissue damage. This is triggered by the formation of antigen antibody complexes also known as immune complexes.

Upon the entry of an antigen, the antibodies bind themselves to the antigen leading to complex formation. During high concentrations of antigen, a number of smaller immune complexes are formed which might not get cleared by the macro molecules. These uncleared immune complexes get deposited into blood vessels, joints and kidneys. The deposited complexes lead to the induction of inflammatory response and cause tissue damage through phagocytosis.

The reaction time in case of type III hypersensitivity may vary from hours to weeks depending on the immunological memory of the antigen. It may take only a few hours if the immune system is pre-sensitized, but it takes few weeks if the antigen is challenged for the first time.

Best example for this type of hypersensitivity is rheumatoid arthritis and farmer's lung disease.