Hypersensitivity

• What is hypersensitivity?

  The production of an excessive or inappropriate immune response causing gross tissue damage when the body meets antigen for the second succeeding times
• When does hypersensitivity occur?

  There are large quantities of antigen, the immune system is at a heightened level, there is a situation where self damage outweighs the defensive advantages
• What is an allergen?

  An antigenic substance that produces hypersensitivity
• What is a type I reaction?

  Anaphylactic
• What is a type II reaction?

  Cytotoxic
• What is a type III reaction?

  Immune complex
• What is a type IV reaction?

  Delayed hypersensitivity
• Why does cell mediated hypersensitivity occur?

  Caused by lymphokines and cytotoxic T cells interacting with antigen
• What reaction is found up a skin test if positive for a type I reaction?

  Wheal and flare reaction
• What mediates type I reactions?

  IgE on mast cells, basophils and IgG
• What can be seen at a cellular level in a type I reaction?

  Vasodilation, oedema, eosinophilia, degranulated mast cells
• How is degranulation of mast cells stimulated?

  Direct injury, cross-linking of immunoglobulin receptors, activated complement proteins
• Which immunoglobulin do mast cells irreversibly bind with?

  IgE
• When do mast cells activate?

  When an allergen binds to IgE
• What does histamine do to the body?

  Dilates post-capillary venules, activates endothelium to increase blood vessel permeability, irritates nerve endings, causes flare and wheal reaction
• What happens to the body in a severe and fatal anaphylactic shock?

  Difficulty in breathing, urticaria, pulmonary oedema, fall in blood pressure
• What are urticaria?

  Hives
• What happens in local anaphylaxis?

  Skin reactions and urticaria
• How does the dose of an allergen affect the response?

  It is not related
• What are some signs of anaphylaxis?


  • Skin of the face and upper chest appears blotchy, flushed, uticaria, pruritis
  • abdominal pain, nausea, diarrhoea
  • chest pain, tight chest, coughing, 0wheezing, difficulty breathing, rapid or weak pulse, palpitations, tachycardia, unconsciousness, cardiac arrest
  • swelling around eyes, tongue, lips pharynx, laryngeal odema, conjunctivitis,
• Which cells mediate type II reactions?

  IgM, IgA
• When do type II reactions occur?

  Transfusion reactions, haemolytic diseases, thrombocytopenia
• What causes a type III reaction?

  Fungal, parasitic, bacterial cells, viruses
• What causes a type II reaction?


  Red blood cells
• What mediates a type III reaction?

  IgG, IgM, IgA - with or without complement system
• What occurs on a positive skin test for type III reactions?

  Erythema and oedema
• What happens in a type III reaction on a cellular level?

  Inflammatory reaction with polymorph infiltration and thrombosis
• Which cells mediate type IV reactions?

  Killer T cells, macrophages, lysosomal enzymes and lymphokines
• What appears in a positive skin test for type IV reactions?

  Erythema and induration