Allergy Flashcards
Allergic disorder
Immunological process that results in immediate and reproducible symptoms after exposure to an allergen.
In clinical practice the immunological process usually involves an IgE mediated type 1 hypersensitivity reaction
Allergen
Usually a harmless substance that can trigger an IgE mediated immune response and may result in clinical symptoms
Sensitisation
Detection of specific IgE either by skin prick testing or in vitro blood tests
Occurs more often than allergic disease
Two main types of TH2 immune responses
Microbial PAMP –> structural features recognition –> TH1, TH17 immune response
Helminthes, allergens, venoms –> functional feature recognition –> TH2 immune response
Immune system recognises enzymatic activates of allergens and multicellular parasites - no direct recognition as seen with bacteria, viruses and fungi
Pathway for TH2 immune response to allergens, worms, venoms
Allergens, Worms, venoms –> Stressed or damaged epithelium –> IL1alpha, IL15, IL33, TSLP –> TH2, TH9, ILC2 –> IL4, IL5, IL9, IL13 –> eosinophils, basophils –> worm and allergen expulsion, mucous secretion
Allergens, Worms, venoms –> Stressed or damaged epithelium –> IL1alpha, IL15, IL33, TSLP –> TFH2 –> IL4, IL21 –> B cells –> IgE, IgG4
Worms, Allergens, Venoms –> mast cell activation –> histamine, prostaglandins, leukotrines, proteases –> endothelium, smooth muscle, neurons –> worm and allergen expulsion, enhanced epithelial barrier function
What are the signalling cytokines in allergic reactions
IL-1alpha
IL15
IL33
TSLP
What are the effector cytokines in allergic reactions
For eosinophils/basophils: IL4, IL5, IL9, IL13
For B cells: IL4, IL21
What are the biological and drug targets in allergic disorders
Histamine Leukotrines IL4 IL5 IL13 IgE IgG4
TH2 immune response features
Epithelial cells and mast cells detect allergens, venoms, and worms
Stressed or damage epithelial cells secrete IL-25, IL-33 and TSLP to act on memory CD4 T cell subsets, innate lymphoid cells and other lymphoid cells to promote secretion of IL-4, IL-5, IL-9, IL-13.
Cytokines secreted by tissue lymphocytes act on effector cells (eosinophils, basophils, epithelial cells, B cells, sensory neurons endothelium and smooth muscle cells) to eliminate and expel pathogens allergens, and repair tissue damage.
Epithelial and mast cells can both detect and eliminate pathogens and allergens.
What route promotes immune tolerance
Oral
What route promotes IgE sensitisation
Skin
What are risk factors for development of IgE antibodies
Defects in skin epithelial barrier (e.g. atopic dermatitis)
What skin features predispose to allergic reactions
Skin dendritic cells (DC) [Langerhans cells and dermal DC promote secretion of TH2 cytokines much more efficiently than other DC subsets which suggest that different DC subsets may prime Th2 immune reponses in humans
What causes IL4 secretion
IL-4 secretion is only induced following peptide-MHC presentation to TCR to either naïve and/or memory Th2 cells
Which one of the following proteins/cytokines is NOT a drug target for current drugs and/or biologics used to treat allergic disorders?A. IL-13
B. Histamine
C. IL-33
D. IgE
E. IL-5
IL33
What allergies onset in infancy
Atopic dermatitis Food allergy (milk, egg, nuts)
What allergies onset in childhood
Asthma (HDM, pets) Allergic rhinitis (HDM, grass, tree pollens)
What allergies onset in adulthood
Drug allergy
Bee allergy
Oral allergy syndrome
Occupational allergy
Most common allergic disorder in adults
Asthma - 10%
What are the theories for why allergic disorders have risen over the last 50-100 years
Hygiene hypothesis: lack of childhood exposure to infectious agents increases susceptibility to allergic diseases by supressing natural development of immune system.
Lack of Vitamin D in infancy is a risk factor for development for food allergy
Dietary factors: reduced omega and linoleic fatty acids, delayed introduction of peanuts in children with egg allergy and atopic dermatitis
Rise in food allergy may be associated with high concentration of dietary advanced glycation end products and proglycating sugars which immune system mistakenly detects as causing tissue damage; fast food and soda
What is the hygiene hypothesis
Lack of childhood exposure to infectious agents increases susceptibility to allergic diseases by supressing natural development of immune system.
Clinical features of IgE allergic responses
Occurs within minutes or up to 3 hours after exposure to allergen and symptoms can include:
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
Vasculature and CNS: symptoms of hypotension (faint, dizzy, blackout) and a sense of impending doom
At least 2 organ systems are usually involved.
Reproducible: occurs after every exposure
Allergic symptoms may be triggered by cofactors such as exercise, alcohol, and possibly infection.
What is used to select what allergens should be tested by skin prick and/or blood tests
Clinical history
What symptoms are NOT associated with IgE allergic reactions
Fatigue
Migraine
Recurrent episodes of abdominal pain, diarrhoea, constipation, bloating
Hyperactivity
Depression
Symptoms may vary over time, with antigen dose and source
Elective investigations of allergic disease
Skin prick tests
Laboratory measurement of allergen specific IgE
Component-resolved diagnostics
Challenge test - supervised exposure to the putative antigen
Investigations of allergic disease during an acute episode
Evidence of mast cell degranulation - serum mast cell tryptase levels
How is a skin prick test carried out
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
What indicated a positive skin prick test
A positive test is indicated by a wheal ≥ 3mm greater than the negative control.
What should be discontinued before a skin prick test
Antihistamines should be discontinued for at least 48 hours beforehand
What is the considered to be the most sensitive and specific test for allergy
Skin prick testing is considered to be more sensitive and specific than blood tests to diagnose allergy in routine clinical practice
Advantages of skin prick tests
Rapid (read after 15 minutes)
Cheap and easy to do
Excellent negative predictive value ( > 95%)
Increasing size of wheals correlates with higher probability for allergy
Patient can see the response
Disadvantages of skin prick test
Requires experience to interpret
Risk of anaphylaxis: 1 in 3000
Poor positive predictive value: high false positive rate
Limited value in patients with dermatographism or extensive eczema
False negative results with labile commercial food extracts
How is a serum specific IgE blood test carried out
Allergen bound to sponge in a plastic cap and patient’s serum is added.
Specific IgE (if present) binds to allergen.
Anti-IgE antibody tagged with a fluorescent label is added.
Amount of IgE/Anti-IgE is measured by fluorescent light signal.
Much more expensive than SPT
Advantages of serum specific IgE test
May help diagnosis of allergy in someone with appropriate clinical history
Higher values are more likely to be associated with allergic disorder and can be used to triage patients who do not need oral food challenges
Results of serum specific IgE do not predict severity of reaction
Very good negative predictive value however lot of false positive ( > 80% of patients with peanut specific IgE are asymptomatic)
Concentration of specific IgE can be used to predict which children may outgrow allergy and should proceed to oral food challenge
Can be used to monitor response to anti-IgE therapy
Indications for specific IgE blood test
Patients who can’t stop anti-histamines Patients with dermatographism Patients with extensive eczema History of anaphylaxis Borderline/equivocal skin prick test results
What is component resolved diagnostics
Blood test to detect IgE to single protein components
What is component resolved diagnostics useful for
Diagnosis of peanut and hazelnut allergy
How does a component resolved diagnostic test work
IgE sensitisation to birch pollen homologue peanut and hazelnut allergen component target heat and proteolytic labile protein and usually associated with minor or no symptoms
IgE sensitisation to seed storage peanut and hazelnut allergen components target heat and proteolytic stable protein and are usually with severe allergic reactions
What is a good biomarker for anaphylaxis
Mast cell tryptase
What is mast cell tryptase
Pre-formed protein found in mast cell granules
Why is mast cell tryptase a good biomaker for anaphylaxis
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 a challenge test used for
Test ofr food and drug allergy