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Flashcards in Immunological Investigations Deck (35)
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Describe a type 1 hypersensitivity reaction.

  • Takes seconds to 30 minutes. 
  • Antibody reaction involving IgE mediated histamine release from mast cells.
  • Basophils also involved. 
  • Histamine causes vasodilation and bronchoconstriction. 
  • Examples:
    • Anaphylaxis
    • Allergic asthma


Describe a type 2 hypersensitivity reaction.

  • Takes minutes to hours.
  • Also known as cytotoxic reactions and IgG and IgM with antibody and complement. 
  • Examples:
    • Drug induced haemolytic anaemia
    • Goodpasture's nephritis


Describe a type 3 hypersensitivity reaction.

  • Takes 3-6 hours.
  • AKA immune complex.
  • Antibody reaction involving IgG and IgM. 
  • Examples:
    • Systemic Lupus Erythematosus

    • Farmer's lung


Describe a type 4 hypersensitivity reaction.

  • AKA delayed type reaction. 
  • T-cell reaction.
  • No antibodies involved.
  • Takes 2-28 days.
  • Examples:
    • Contact dermatitis
    • Leprosy
    • Granulomas
    • Tuberculin test


Describe atopy.

  • A genetically determined capacity to make IgE class antibodies to allergens commonly encountered in the general environment.
  • This is associated with allergies of the immediate type and the clinical syndromes of rhinitis, asthma and atopic eczema. 
  • Patients with atopic eczema are more likely to get immediate and delayed type allergies but most patients with atopic eczema do not have a clear allergic driver for the disease. 


What is the diagnostic test of choice for supporting a diagnosis of type 1 allergy?

Skin prick test


Describe how a skin prick test is thought to work.

  • SPT utilises the presence and degree of cutaneous reactivity as a surrogate marker for sensitsation within target organs. 


What materials are needed to perform a skin prick test?

  • A patient with a history c/w type 1 allergy
  • A lancet
  • A skin prick test kit comprising allergens you are interested in plus a positive control (histamine) and a negative control (dilutent).
  • Food for prick to prick testing
  • Full resus facilities (severe reactions uncommon)


Describe how to interpret the results of a skin prick test.

  • A positive skin prick test only confirms sensitisation to a specific allergen - clinical relevance must be interpreted based on the medical history and clinical symptoms. 
  • A positive result is usually regarded as being a wheal ≥3mm greater than the negative control.
  • Wheal size does not necessarily correlate with severity of allergy.
  • Reaction to negative control could be dermographism. 
  • No reaction to the positive control could be due to antihistamines.
  • SPT is less reliable with food allergens than with inhaled allergens. 


What are the pros of skin prick testing?

  • Relatively painless
  • Low risk of side effects
  • Cheap 
  • Reproducible 
  • Real time results to demonstrate to patient
  • False negatives are uncommon if done properly


What are the cons of skin prick testing?

  • Full resuss facilities should be available (especially when testing for food / latex). 
  • Antihistamines have to be stopped 48 hours before testing.
  • Itch can be uncomfortable.
  • Interpretation difficult in patients with active eczema. 


Describe the use of serum specific IgE antibody concentrations.

  • A complementary tool used to diagnose type 1 allergy, especially in subjects who cannot undergo skin prick tests
  • There are multiple different assays:
    • RAST
    • CAP-RAST
    • ELISA


What are the pros of testing serum specific IgE antibody concentrations?

  • Not influenced by current therapy (antihistamines)
  • Can be performed when patient has widespread skin disease
  • Completely safe
  • Evolving knowledge/technologies


What are the cons of testing serum specific IgE antibody concentrations?

  • Painful (venepuncture)
  • Expensice 
  • Results can take weeks 
  • Difficult to interpret, especially in the context of high Total IgE (atopy) - titre is important


Describe the different methods of food testing.

  • Double-blind, placebo-controlled food challenge.
    • Gold standard method for diagnosing food allergy.
  • Food withdrawl and reintroduction.
    • Best test for diagnosing non-IgE mediated food allergy.
    • Very crude, not validated. 


Describe why patch testing would be used.

  • A test for delayed hypersensitivity (type 4, cell mediated)
    • A reaction in the skin also known as allergic contact dermatitis.


Describe how a patch test is thought to work.

  • If a patient is allergic to the tested material, their CD8 lymphocytes are sensitised, and contact with the offending allergen will cause them to secrete certain cytokines which will attract inflammatory cells to the site of contact and produce a positive rection.
  • This manifests as erythema, papules and vesicles. 


Describe what happens during the process of patch testing.

  • Patient history and decision on which 'series' to test. 
  • Day 1 - patches placed and map made. 
  • 48 hours - patches removed and patient examined.
  • 96 hours - final patch test reading and results given. 


What are the pros of patch testing?

  • Safe 
  • Real time results to demonstrate to patient 
  • Well validated


What are the cons of patch testing?

  • Uncomfortable 
  • Time consuming 
  • Interpretation requires experience (risk of false positives)
  • Can throw up incidental reaction unrelated to clinical problem
  • Not possible in patients with widespread eczema 
  • Angry back


What is serum tryptase?

  • Mast cell tryptase is a measurable enzyme released by mast cells when they degranulate. 
  • Acute elevation of serum tryptase indicates widespread degranulation of mast cells. 
  • Does not delineate whether or not this is via IgE-mediated mechanism or otherwise. 


Describe how a serum tryptase test would be done and what it identifies. 

  • Useful in the assesment of suspected anaphylaxis, especially when it is not clear cut, for example when there is no rash / angio-edema, no clear trigger. 
  • Does not identify the cause of the reaction.
  • Timed samples (1) ASAP from presentation and (2) within 1-2 hours (no greater than 4).
  • If elevated, need to chack baseline when well (>24 hours after the event).


In what situations would allergy testing NOT be indicated?

  • Allergy 'screening'
  • Chronic urticaria / angiooedema
  • Food intolerances
  • Routine childhood atopic eczema
  • Routine asthma and hayfever
  • Unexplained symptoms


  • Presenting patient:
    • 3 year old boy
    • Atopic eczema
    • Dad feels there must be something driving it and would like allergy testing. 
  • What test, if any, should be carried out?

No test


  • Presenting patient:
    • 22 year old female admitted with collapse.
    • Widespread hives + lip swelling.
    • History of possible bite or sting, but didn't see offending insect.
    • Responds to resuscitation + IV chlorphenamine + IV hydrocortisone.
  • What test, if any, should be carried out?

  • Serum tryptase immediately.
  • Further down the line perhaps serum specific IgE to venom.


  • Presenting patient:
    • 3 month old boy
    • Severe atopic eczema, especially face
    • Poor weight gain and green stools
  • What test, if any, should be carried out?

  • Remove cow's milk from diet for 4-6 weeks, and if the eczema improves, reintroduce it to prove the point. If the eczema worsens again after reintroduction you can say for sure it is a cow's milk allergy. 
  • Kids often grow out of this. 
  • Initially give lots of emollients and potent topical steroids.


  • Presenting patient:
    • 7 year old boy
    • History of atopic eczema and asthma, both well-controlled.
    • Presents with summertime sneezing and itchy eyes.
  • What test, if any, should be carried out?

  • No test.
  • Give antihistamines


  • Presenting patient:
    • 18 month old girl
    • History of atopic eczema, quite poorly controlled. 
    • Mum reports facial itch and erythema approximately 1 hour after consuming scrambled egg. 
      • Never happened before 
    • Has always avoided nuts as Mum is worried about peanut allergy. 
  • What test, if any, should be carried out?

Skin prick test


  • Presenting patient:
    • 35 year old male 
    • Lifelong asthma 
    • New job in bakery since-when asthma control has deteriorated. 
    • Seems to be better when on leave. 
  • What test, if any, should be carried out?

Skin prick test (there is a series of skin prick tests under 'occupational asthma'). 


  • Presenting patient:
    • 55 year old man in hospital with community acquired pneumonia.
    • Presents with widespread maculopapular rash 4 days after starting IV co-amoxiclav.
  • What test, if any, should be carried out?

  • Patch test. 
  • This will prove allergy rather than just a bit of contact dermatitis.