IMMUNO: Allergy Flashcards
What is the difference in triggers of Th1/Th17 and Th2 immune responses?
Th1/17 is the immune reponse to bacteria/fungi/viruses
Th2 is to parasites, venoms and allergens.
Name the signalling an effector cytokines in Th2 immune memory responses. What are the sensors and effectors in this reaction?
Give an overview of the Th2 immune responses.
- Stressed or damaged epithelium will release signalling cytokines (e.g. TSLP / Thymic Stromal Lymphopoietin)
- Cytokines act on Th2, Th9 and ILC2 (Innate Lymphoid Cells T2) à promote the section of IL4, IL5 and IL13
- IL-4 acts on both pathways
- IL-4 stimulates B-cells to produce IgE and IgG4
- These then act on eosinophils and basophils which plays a role in the expulsion of parasites and allergens but can also contribute to tissue injury
- The TSLP and other cytokines released by the damaged epithelium can also activate follicular Th2 cells which then releases IL4 (n.b. cDC2 = Classical Dendritic Cell 2)
Why have allergic disorders risen over the past 150 years?
Many theories:
Hygiene hypothesis: lack of childhood exposure to infectious agents increases susceptibility to allergic diseases by supressing natural development of immune system.
Increase in epithelial-damaging agents linked to industrialisation, urbanisation and modern lifestyle.
Loss of symbiotic relationship with bacteria with reduction in biodiversity and alteration in composition of composition of gut skin and respiratory bacteria
Other factors:
- Shift to indoor lifestyle and increased exposure and sensitisation to perennial indoor allergen (asthma)
- Change in food processing and preparation
- Delayed introduction of peanuts in children with egg allergy and atopic dermatitis
What is throught to be the cause behind allergic rhinitis?
Link with pollen made in 1870 however still a rare disease although by 1900 well recognized in higher income groups and epidemic in New York in 1940.
Clean water, improved food hygiene, eradication of helminths, limited exposure to farm animals and soil bacteria smaller family size, (all components of hygiene) were achieved by 1920’s in major urban centres
Changes in farming practices late 19th century: increased dairy herds, introduction of rye grass ( generate more pollen) important for rise in grass pollen rhinitis
Give a piece of evidence that microbial environment can protect against asthma.
Comparing Amish and Hutterite communities
- They share similar genetic background yet have different lifestyles
- Prevalence of asthma and sensitisation much lower in Amish than in Hutterites
- Increase in LPS in dust samples collected from Amish than Hutterites
- Increased secretion of innate immune cytokine by PBMC exposed to LPS in Amish than Hutterites
- Dust samples from Amish suppress allergic inflammation in a murine asthma model
When did food allergy rise? What is the difference in reaction to roast and boiled peanuts?
1990 onwards: Significant increase in number of cases of peanut allergy attending allergy clinics in the UK and the USA
Early oral exposure will protect against development of peanut allergy
Sensitisation to peanut and wheat can occur from exposure through the skin
Differences in preparation of peanuts (roast promotes IgE whereas boiled IgG)
Epidemic of delayed food allergy to red meat observed in SE USA for last 10 years: increasing case in France, Germany, Australia but not in UK to date)
How is allergic disease diagnosed?
History is the key to diagnosis
Examination
Allergen specific IgE (Sensitisation) Tests - (1) Skin prick and intradermal test (2) IgE blood tests
Functional allergen tests
In vitro tests
- Basophil activation
- Serial mast cell tryptase
Ex vitro tests
- Open or blinded allergen challenge
What is the difference in allergy types with age?
Infants
- Atopic dermatitis
- Food allergy (milk, egg, nuts)
Childhood
- Asthma (HDM, pets)
- Allergic rhinitis (HDM, grass, tree pollens)
Adults
- Drug allergy
- Bee allergy
- Oral allergy syndrome
- Occupational allergy
What are the clinical features of IgE allergic responses?
Occurs within minutes or up to 3-4 hours after exposure to allergen
SYMPTOMS
- Skin: angioedema (swelling of lips, tongues, eyelids) , urticaria ( wheals or ‘hives’), flushing and itch
- Respiratory tract: cough, SOB wheeze, sneezing, nasal congestion and clear discharge, red itch watery eyes
- Gastrointestinal tract: nausea, vomiting and diarrhoea
- Blood vessels and Brain: symptoms of hypotension (faint, dizzy, blackout) and a sense of impending doom
What other cofactors can help induce anaphylaxis?
nexercise, alcohol, NSAID and in children viral infection
Where can link between exposure and onset of symptoms not be obvious?
- House dust mite
- Fungal and Staph skin colonisation
- Red meat ingestion
What is a clinical feature of allergy upon rexposure?
- At least 2 organ systems are usually involved.
- Reproducible: occurs after every exposure
Is IgE presence necessary for diagnosing allergic disease? What marker in addition is better for diagnosing allergic disease?
Necessary but not suffifient for its diagnosis
IgE is best considered a risk factor for allergic disease as it is most commonly associated with sensitisation which is generally more common
In the context of an appropriate history larger skin wheals and higher specific blood IgE values are more likely to be associated with allergic disorder
Do results of skin prick or IgE in serum predict severity of reaction?
No
How is a skin prick test done? What should be done beforehand?
- Expose patient to standardised solution of allergen extract through a skin prick to the forearm.
- Use standard skin test solutions and positive control (histamine) and negative control (diluent)
- Measure local wheal and flare response to controls and allergens
- IgE crosslinking on skin mast cells, leading to degranulation and release of histamine and other inflammatory mediators
- A positive test is indicated by a wheal ≥ 3mm greater than the negative control.
- High positive and negative predictive and positive skin for aeroallergens
- Allergen extracts labile for some fruit and vegetables: : prick-prick test: food and SPT
Antihistamines and some anti-depressants should be discontinued for at least 48 hours beforehand
What are the advantages and disadvantages of skin prick testing?
What is the use of intradermal tests in allergy? How does it compare to SPT?
Application of positive, negative controls and allergens into the skin
More sensitive but less specific than SPT
Best used to follow up negative venom and drug allergy test (better than blood tests)
Can be used if SPT to allergen is negative but convincing history
Labour intensive, greater risks of anaphylaxis
How do sensitisation blood tests work?
3 major providers (Phadia, Siemens, Hycor); all assays have excellent technical performance; intra-assay CV < 15% and inter-assay CV < 20%
Lower limit of detection 0.1kUA/L although clinical significance of low IgE antibody concentration remains to be determined
Results from three main testing platforms are not interchangeable: very good agreement on the presence or absence sensitisation however quantitative levels for specific allergens vary widely
How can serum IgE predict allergic symptoms?
Risk profile of serum IgE for prediction of allergic symptoms
- Concentration: higher levels more likely to be associated with symptoms
- Molecular target within whole extract or even individual epitope can be linked to symptoms
- Affinity (strength of binding) to target: higher affinity associated with risk
- Capacity of IgE antibody to induce mast/basophil degranulation
What are the indications for doing blood sensitisation tests?
- No access to SPT and/or IDT
- Patients who can’t stop anti-histamines
- Patients with a history of dermatographism, extensive eczema
- Patient with a history of anaphylaxis
- Decision on who needs food challenge
- Prediction for resolution of egg, milk, wheat allergy
- Monitor response to anti-IgE therapy
What does component resolved/molecular allergen testing involve? What is its diagnostic use?
Test for IgE sensitisation against individual protein within whole allergen extract
Increasing use of over last 10 years
Second line: reflex test for positive whole allergen blood tests
Diagnostic use
- Food allergy (nuts, egg, milk)
- Insect allergy (wasp and bees)
- Guide to immunotherapy (grass and HDM)
Give 5 examples of food component allergy tests
Nuts
- Storage proteins (2 s albumins): severe reactions
- Pathogenesis-related proteins (PR10 bet v1 homologue) which are also found in tree pollens, fruits and vegetables: mild reaction
- Non specific lipid transfer proteins which are also found in in fruit (peach) , tree pollen (plane), vegetables and legumes ( can be mild or severe)
Wheat - Omega-5-gliadin bettermarker of specific wheat allergy than total wheat IgE
Egg and milk - Heat stable proteins ovomucoid (egg) and caseins ( estimate who outgrow allergy)
Fish and shellfish - Parvalbumin in fish, Tropomyosin in crustaceans (cross reactive)
What is the use of measuring mast cell tryptase in allergy?
- Tryptase: pre-formed protein found in mast cell granules
- Systemic degranulation of mast cells during anaphylaxis results in increase in serum tryptase
- Peak concentration at 1-2 hours; returns to baseline by 6-12 hours
- Failure to return to baseline after anaphylaxis may be indicative of systemic mastocytosis
- Useful if diagnosis of anaphylaxis is not clear (hypotension + rash during anaesthesia)
- Reduced sensitivity for food induced anaphylaxis
What is the basophil activation test and its use?
- Measurement of basophil response to allergen IgE cross linking
- Activated basophils increase the expression of CD63, CD203, CD300 protein on cell surface
- Increasing use in diagnosis of food and drug allergy: surrogate marker for challenge tests
- Efforts to standardise test to use by diagnostic laboratories to reduce need for challenge tests
What is the gold standard for food and drug allergy diagnosis?
Food challenge
What is the use of challenge testing?
- Gold standard for food and drug allergy diagnosis
- Increasing volumes of the offending food/drug are ingested
- Double blind placebo or open challenge
- Food challenges take place under close medical supervision. Very expensive in terms of clinical staff time.
- Can be difficult to interpret mild symptoms
- Risk of severe reaction
Define food allergy and food intolerance.
Food allergy: adverse health effect arising from specific immune response that occurs reproducibly on exposure to a given food
Food intolerance: non immune reactions which include metabolic, pharmacological and unknown mechanisms
Food allergy likely affect up to 5% of adults and 8% of children
List 3 types of adverse reactions to foods.
Food intolerance - food poisoning (bacterial, scromboid toxins), enzyme deficiencies (lastase), phaarmacological (tyramine, caffeine)
Food aversion e.g. fads, eating disorders.
Food allergy - IgE mediated anaphylaxis, mixed IgE and cell mediated (atopic dermatitis), non-IgE mediated (coeliac), cell mediated (contact dermatitis)
List the 6 most common food allergies. Are these outgrown? What is a risk factor for food allergy?
What are the challenges in clinical history of food allergies?
- What does the patient mean by allergy.
- Distinguish between IgE and non IgE mediated symptoms.
- Dose, how food is prepared and co-factors can influence clinical symptoms
- Does the patient have any history of atopic disease
- Enquire about previous investigations for food allergy ie SPT, IgE blood tests and complementary medical tests
- Has elimination of food made any difference to symptoms
- Consider other differential diagnoses (food intolerance, eating disorders, coeliac disease)
List the investigations to use for food allergy.
- Clinical history is used to estimate prior probability of allergy, identify culprit foods and decide what diagnostic allergen tests are used to achieve a post test probability of allergy
- A positive SPT/specific IgE blood test is useful to confirm a clinical history of food allergy.
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A negative SPT/specific IgE blood test essentially excludes IgE mediated allergy (Negative predictive value NPV = 95%).
- A positive skin test or food specific IgE blood test indicates sensitisation but not necessarily allergy
- Fruit and vegetable skin prick test solutions are labile and it often better to use actual fruit or vegetable.
- Increasing high food-specific IgE levels or larger skin tests wheal size indicate a higher chance of allergy
- IgE concentrations and SPT wheal sizes to determine presence/absent allergic disease and/or disease persistence/resolution vary with age, hospital and different blood test assays.
- Testing for individual allergen protein component can distinguish between IgE sensitisation and IgE mediated allergy
- Gold standard for the diagnosis of food allergy is a double blind oral food challenge.
Results can be followed over time to monitor for allergy persistence or resolution
What is the management of food allergy?
Avoidance
- Education about food labelling, interaction with restaurants, school
- Nutritional input for dietary balance, growth in children
- Acknowledge anxiety, potential bullying: mental health support if needed
Emergency management
- Anaphylaxis guidelines
- Ensure allergic asthma is well controlled
Prevention
- Breast feeding: strong family of allergy
- LEAP study: early rather than delayed introduction of peanut in high risk children (moderate/severe AD and egg allergy) significantly reduces development of peanut IgE sensitisation and allergy
Give an example of food-induced anaphylaxis to meat.
- Symptoms occur 3-6 hours after eating red meat and gelatin
- IgE antibody to oligosaccharide alpha-gal (α1, 3-galactose) found in gut bacteria
- Induced by tick bites which should be avoided
What are the most common triggers for anaphylaxis?
Peanut, tree nut shellfish, fish, milk and eggs are most common
Natural history dependent on food
Which foods are associated with exercise induced anaphylaxis?
Food induces anaphylaxis if individual exercises within 4-6 hours of ingestion
Common food triggers are wheat, shellfish, celery
What are the features of oral allergy syndrome?
Limited to oral cavity, swelling and itch: only 1-2% cases progresses to anaphylaxis
Sensitisation to inhalant pollen protein lead to cross reactive IgE to food
Onset after pollen allergy established: affect adults > young children
Respiratory exposure to pollen (birch) results in IgE directed to homologous proteins in stone fruits (apple, pear) vegetables (carrot) and nuts (peanut, hazelnut)
Cooked fruits, vegetables and nut cause no symptoms: heat labile allergens detected by component allergen tests
Serial mast cell tryptase
