The immune system can sometimes overreact, causing moderate to severe discomfort. This article is a brief overview of the four types of hypersensitivity.

Type 1

This is an immediate hypersensitivity response due to exposure to an allergen. The person may start swelling or rash may appear soon after exposure. This is due to activation of mast cells which releases histamine. Histamine causes blood vessels to become more porous, hence releasing more fluid to the area along with white blood cells. In serious cases, a person reacting to an allergen may have a sudden drop in blood pressure causing lack of oxygen to critical organs. This is called an anaphylactic response and may require immediate injection with an EpiPen. EpiPen contains the hormone epinephrine also known as adrenalin to increase blood pressure and open airways in the lungs. Examples of type 1 hypersensitivity includes food allergies and allergic rhinitis.     

Type II

Type II hypersensitivity is also called cytotoxic hypersensitivity. It occurs when antibodies attacks antigens that are attached to the surface of cells in the body. This leads to destruction of the cell by

  1. Activation of the complement system leading to cell lysis
  2. Attracting phagocytes to destroy the cell
  3. Attracting granulocytes to the area, releasing toxic substances that destroys the cell

Type II sensitivity occurs as a response to getting the wrong blood type transfusion, an non-compatible organ transfer, and or certain medications that may attach to cell surface

Type III

This type of hypersensitivity is called immune complex hypersensitivity. It occurs when antibody-antigen complexes deposit on tissue walls. This attracts white blood cells to the area for the purpose of removing them. Since the complexes are attached to body cells, those cells become casualties in the process.

Type IV

This type of hypersensitivity is called T cell-mediated or delayed hypersensitivity since there is a long delay between exposure and reaction. Remember from your study of how the immune system works that antigen-presenting white blood cells digest antigens and then present themselves on their surfaces for T-Cells to bind. The activated T cells in turn activate other immune cells. This takes time which explains why the symptoms of Type IV hypersensitivity is not felt immediately. An example is what happens when you are exposed to poison ivy. At first you will feel nothing. But, about 12-48 hours later, you will be covered with an itchy rash.

Another example is the tuberculosis test, also called Mantoux tuberculin skin test. In the test, the antigen tuberculin, a protein from the tuberculosis bacterium is injected under the skin. If the person has been been exposed to tuberculosis or has the disease, antibodies would be in their body. This would cause a reaction resulting in raised and thickened skin known as an induration. If there was never any exposure, there would be no antibody present to cause the reaction and so there would be no induration.

Courtney Simons
Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University.
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