Hypersensitivity

What is hypersensitivity?

Hypersensitivity, as the name suggests, is the excessive or inappropriate activation of the immune system due to exposure to exogenous or endogenous antigens. When a reaction takes place, inflammation occurs and there is also some tissue and cell damage.


Antibodies responsible for hypersensitivity reactions:
  • IgE
  • IgD
  • IgA
  • IgM
  • IgG
There are four types of hypersensitivity:

TypeⅠ: Intermediate Hypersensitivity

Immediate hypersensitivity involves the antibody IgE, with examples being allergies and anaphylaxis. Evidence of the reaction begins within minutes of the antigen challenge, after the antigen is inhaled, ingested, injected or comes into contact with skin. Examples of this include:
  • Atopic dermatitis (eczema)
  • Allergic rhinitis (hayfever)
  • Asthma
  • Food reactions
  • Anaphylaxis (widespread vasodilation and oedema)
Steps in the reaction of IgE against a foreign antigen are outlined below:
  1. Initial antigen exposure to the B lymphocytes
  2. B cells differentiate into plasma cells
  3. Plasma cells produce IgE antibodies for the specific antigen
  4. IgE antibodies are bound to mast cells and basophils
  5. Mast cells and basophils are 'sensitised'
  6. B lymphocytes are again exposed to the antigen, either months or years after the initial exposure
  7. Antigen combines with the IgE antibodies on the cell membrane of basophils and mast cells
  8. Antigen-antibody reaction
  9. Basophils and mast cells rupture
  10. Release of chemical mediators (Histamine) from basophils and mast cells
  11. Vasodilation of arterioles and venules, as well as increased capillary permeability (initial phase response)
  12. Fluid moves from the blood into the tissues: mucosal oedema in bronchi, epithelial damage and leukocyte infiltration (late phase response)
  13. Blood volume decreases
  14. Blood pressure decreases
  15. Shock

Type Ⅱ: Cytotoxic Hypersensitivity

Cytotoxic hypersensitivity is when the body produces an antibody after being exposed to an antigen which then attacks the antigens on the body's own cells. A common example is when an incompatible blood type is used in a blood transfusion or in myasthenia gravis (weakness and rapid muscle fatigue under voluntary control).

Type Ⅲ: Immune Complex Hypersensitivity

Immune complex hypersensitivity occurs when there is an accumulation of antigen-antibody complexes by specific 'cleaning up' cells, giving rise to inflammation and leukocyte infiltration. An example of this type of hypersensitivity is Systemic Lupus Erythematosus (immune system attacking its own tissues).

Type Ⅳ: Cellular (Delayed) Hypersensitivity

Unlike the others, this type of hypersensitivity is cellular-mediated, which also means it can take up to several days to have effect. An example includes tuberculosis (bacterial infection mainly affecting the lungs).



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