Allergy Flashcards
What type of hypersensitivity reaction is allergy?
Type 1 (IgE mediated)
What is atopy?
General tendency to produce specific IgE Abs on exposure to common environmental Ags.
What is oral allergy syndrome?
Cross-reactivity from pollen allergens to other tings like raw fruit/vegetables.
How can you distinguish between true allergy and oral allergy syndrome?
- Peanut storage protein (Ara h1-3) present in true allergy.
- Pollen cross-reactive peanut protein (Ara h8) present in oral allergy syndrome.
What is the treatment for oral allergy syndrome?
Oral histamine (not EpiPen)
What is this describing?
IgE mediated degranulation of mast cells after exposure to: cow’s milk, egg, peanut, tree nut, fish, prawns.
True food allergy
What is this describing?
Direct stimulation of mast cells or histamine ingestion. Histamine release by bacterial action. Symptoms mimic an allergic reaction.
False food allergy
What is this describing?
Adverse reaction to food with no histamine related symptoms.
Food intolerance - lactose intolerance, gluten insensitivity
What is food sensitivity?
Umbrella term for: true and false food allergy and food intolerance.
What conditions is atopy strongly associated with?
Asthma, allergic rhinitis, eczema.
What are the steps of sensitisation before an allergic reaction?
- Exposure to allergen.
- APC present Ag to Th2 cells
- Th2 produces IL-4, IL-13 to stimulate B cells to make IgE.
- IgE binds to FC receptors on mast cells.
What are the steps of an allergic reaction?
- On next encounter of allergen, Ag binds to the IgE.
2. Crosslinking activates the mast cell, release of pre-formed and newly generated mediators.
What are the pre-formed and newly generated mediators in allergic reaction?
- Pre-formed - histamine, tryptase, heparin.
2. Newly generated - leukotrienes, prostaglandins
What does histamine cause in allergic reaction?
- Pruritis
- Vasodilation and leakage of fluids - hives, angioedema, drop in BP.
- Smooth muscle contraction - bronchospasm, wheezing.
What is the effect of histamine on these cells?
- Endothelial
- Smooth muscle
- Mucosa
- Increased vessel permeability
- Constriction of airway
- Increased secretion of mucus
How is an allergy diagnosed?
- Clinical history - time course, triggers, seasonality, location, food or aeroallergens
- Allergens for testing selected on the base of history - skin prick, specific IgE, total IgE (least useful).
What does a positive result of specific IgE in allergy confirm?
IgE sensitisation, may/may not be associated with clinical allergy (need bloods and symptomatic Hx for the diagnosis)
What are the important features of skin prick testing?
- Need a negative and positive control
- Wheal >3mm is positive
- High compliance (easy to perform)
- Antihistamines must be stopped 5d prior to test.
- Not advised to patients with extensive eczema.
What are the steps of the oral food challenge?
Only if skin prick test inconclusive:
- Given graded exposure to suspect food under controlled conditions.
- Double blind placebo-controlled food challenges are gold standard (except in Hx of anaphylaxis)
- Resus equipment needs to be on hand.
What is this describing and how is it treated?
Low grade mild infection which activates immune system. Release of histamine in a chronic, random way. Not an allergy.
- Chronic spontaneous urticaria
2. High dose antihistamines, montelukast.
What is the management of a confirmed allergy?
- Avoid food allergen
- Administer emergency treatment if necessary.
- Oral non-sedating, long acting antihistamine (loratadine, cetirizine).
- Aqueous steroid nasal spray (beclomethasone)
- Cromoglicate eye drops/olopatadine eye drops
- Pollen avoidance measures
- SC pollen immunotherapy if medical treatment fails.
