Immunological Investigations Flashcards
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.