Allergic Disorder Flashcards
Define hypersensitivity
objectively reproducible symptoms or signs following exposure to a defined stimulus (e.g. food, drug, pollen) at a dose that is usually tolerated by most people
Define allergy
a hypersensitivity reaction initiated by specific immunological mechanisms. This can be IgE-mediated (e.g. peanut allergy) or non-IgE- mediated (e.g. coeliac disease)
What are the two phases of IgE mediated allergies
Early phase: histamine and mast cell mediator release within minutes of exposure
→ Causes urticaria, angioedema, sneezing, vomiting, bronchospasm, and/or cardiogenic shock
Late phase: may occur 4-6 hours (especially for inhalant allergens)
→ Causes nasal congestion, cough, bronchospasm
IgE vs non-IgE mediated allergy
IgE
Follows exposure and sensitisation to trigger food allergen(s) with the development of serum-specific IgE antibody
Accounts for the majority of life threatening allergic reactions
Produces immediate and consistently reproducible symptoms which may affect multiple organs including GIT, respiratory, CV and neurological systems
Non-IgE
Cell-mediated (T cell) mechanisms
Tends to occur in young children
Give examples of common allergens
Inhalant allergens, e.g. house-dust mite, plant pollens, pet dander and moulds
Insect stings/bites, drugs, and natural rubber latex.
Ingestant allergens, e.g. egg, cow’s milk, nuts, wheat, seeds, legumes, seafood and fruits
What are the risk factors for allergy
PMHx atopy or anapylaxis
FMHx of atopy (asthma, eczema, rhinitis)
Occupation, recreational, geographical exposure to allergens
Hygiene hypotheses (lower risk of allergy in children of large families and those raised in farms, modern living conditions = increased allergy)
What are the symptoms of IgE mediated allergy
Urticaria
Facial swelling
Anaphylaxis within 10-15 minutes
GIT: N&V, diarrhoea, colicky abdominal pain
ENT: nasal itching, sneezing, rhinorrhoea
Respiratory: cough, chest tightness, wheeze, mouth breathing
Sleep: Hx apnoea/snoring
What are the symptoms of non-IgE mediated allergy
GIT: N&V, diarrhoea, colicky abdominal pain, blood in stools (Proctitis, severe vomiting)
What are the signs of allergy on examination
Obs
Height and weight + growth charts
General: urticaria, facial swelling, sneezing, rhinorrhoea, eczema
ENT: mouth breathing, allergic salute, pale and swollen inferior nasal turbinates
Resp: cough, wheeze, hyperinflated chest, harrison sulci
Eyes: conjunctivitis → Denni-morgan folds (prominent creases), blue-grey discolouration below the lower eyelids
What investigations are done for allergy
Serum-specific IgE allergy testing (RAST): raised response
Skin prick testing (ensure antihistamines are stopped)
Double-blind placebo controlled food challenge (must be done in hospital)
What is the management for allergies
Avoidance of relevant allergen
Individualised written allergy management plan
Education: allergy UK, british dietetic association
Review annually
Mild reactions: PO non-sedating antihistamines PRN
Proteins with an unstable tertiary structure may be rendered non-allergenic by heat degradation or other forms of processing e.g. milk, eggs
What is the difference between oral and skin/resp exposure to allergens
Oral exposure → promotes immune tolerance (Due to T-regs from GI mucosa inhibiting IgE synthesis)
Skin and respiratory exposure → induces IgE sensitisation
What constitutes sensitisation testing
Allergen-specific IgE: skin prick or IgE RAST
IgE: positive control (histamine) and negative control dilutent)
- Positive = wheal >3mm than negative control
- Discontinue antihistamines 48h beforehand
IgE RAST (radioallergosorbent) blood tests
- Pt serum added to the allergen → detection of IgE via anti-IgE Abs tagged with a label
Note: tests for sensitisation, NOT clinical allergy
What are the indications for IgE RAST testing
Patients who can’t stop antihistamines (otherwise do skin test)
Patients with dermatographism
Patients with extensive eczema
History of anaphylaxis
Borderline/equivocal skin prick test results
