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Flashcards in Hypersensitivity and Anaphylaxis Deck (10)
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Define hypersensitivity

An inappropriate and excessive immunological reaction to an external antigen due to dysfunctional control of the immune system

Allergy= local reaction

Anaphylaxis = systemic reaction 


Define allergen

An antigen capable of producing a hypersensitivity reaction


Give an example of type 1 hypersensitivity reactions.

What are type 1 reactions mediated by?

Anaphylaxis, allergies, asthma, atopy.

Mediated by IgE and mast cells



Describe the pathology of type 1 hypersensitivity reactions

  • First exposure: 
    • Antigen activation of TH2 cells and stimulation of B cell IgE class switching.
    • IgE production from B cells
    • IgE binds to mast cells
  • Second exposures:
    • Activation and degranulation of mast cells (release of mediators)  
      • Vasoactive amines and lipid mediators → immediate hypersensitivity reaction 
      • Cytokines → late phase reaction 


Describe the process of mast cell degranulation: which substances are released and what effects do they have?

Second exposure of antigen = activation of mast cells = release of mediators (degranulation):

  • Exocytosis of granules with pre-formed mediators:
    • Vasoactive amines → vascular dilation, smooth  muscle contraction
    • Proteases → tissue damage
  • Lipid mediators (secretion)
    • Prostaglandins → vascular dilation
    • Leukotrienes → smooth muscle contraction
  • Cytokines (secretion)
    • E.g. TNF → inflammation, leukocyte recruitment


Describe the clinical features of a type 1 hypersensitivity reaction

  • Airway and eye mucous membranes →pruritis and sneezing, rhinorrhoea and lacrimation.
  • Skin →pruritis and urticaria 
  • Oral and GI mucous membranes → pruritis and angioedema
  • Systemic hypersensitivity → 
    • Local swelling
    • Vasodilation
    • Flushed skin
    • Dizziness
    • Dyspnoea
    • Wheeze
    • Chest-tightness
    • Peri-oral paraesthesia (esp. food)
    • Pale
    • Sweating
    • Hypotension
    • Low GCS


Give examples of type IV hypersensitivity reactions. What are they mediated by?

What may be seen clinically?

Normal cytotoxic T cell response (slow and specific) but excessive and inappropriate. More localised than type I

e.g. contact dermatitis from nickel and other metals, latex

Clinical features: slowly developing, localised reactions 


What investigations can be carried out to prove type I hypersensitivity reactions?

What investigations can be carried out to identify the specific allergen (for type I and IV reactions)?

To confirm type I: measurement of blood markers such as tryptase, IgE, eosinophils. 

To identify specific allergen: skin prick testing; +ve result =  lesion >3mm larger than negative control. 


How can hypersensitivity reactions be treated?

  • Avoidance
  • Mast-cell stabilisers
  • Anti-histamines (blockage of histamine receptors)
  • Steroids (anti-inflammatory)
  • Leukotriene receptor antagonists
  • De-sensitisation (exposure) 


How should anaphylaxis be treated?

  • Airway, breathing, circulation
  • Lie pt down
  • High flow O2
  • IV fluids
  • Adrenaline
  • IV chlorphenamine
  • IV hydrocortisone
  • Salbutamol nebs
  • Rpt adrenaline if no effect after 5 mins