Allergy Flashcards
(25 cards)
What is Allergy?
Type I Hypersensitivity Reaction (IgE Mediated) of the immune system to allergens.
What is an Allergen?
A protein that the immune system recognises as foreign and potentially harmful, leading to an allergic immune response.
What is Atopy?
A predisposition to having hypersensitivity reactions to Allergens - the tendency to develop conditions e.g. eczema, asthma, hay fever, allergic rhinitis and food allergies.
Theory of Allergy.
Skin Sensitisation Theory -
- Break in Infant’s Skin allows Allergens from environment to react with the immune system.
- No contact from the GI tract - absence of GI exposure to the allergen.
What inflammatory mediators are released during Mast Cell Degranulation? (3B)
A : Pre-Formed (Rapid Release) :
- Histamine.
- Tryptase.
- Heparin.
B : Synthesised (Slow Release) :
- Leukotrienes.
- Prostaglandins.
How are Hypersensitivities classified?
Coombs and Gell Classification.
What is a Type I Hypersensitivity Reaction?
IgE antibodies to a specific allergen trigger mast cells (degranulation) and basophils to release Histamines and other Cytokines to cause an immediate reaction.
What is a Type II Hypersensitivity Reaction?
IgG and IgM antibodies react to an allergen which causes activation of Complement System and direct damage to Local Cells.
Examples of Type II Hypersensitivity Reactions (2).
- Haemolytic Disease of the Newborn.
2. Transfusion Reactions.
What is a Type III Hypersensitivity Reaction?
Immune complexes accumulate and damage local tissues.
Examples of Type III Hypersensitivity Reactions (3).
- SLE.
- Rheumatoid Arthritis.
- Henoch-Schonlein Purpur.
What is a Type IV Hypersensitivity Reaction?
Cell-mediated hypersensitivity reaction caused by inappropriate activation of T-cells, causing inflammation and damage to local tissues.
Examples of Type IV Hypersensitivity Reactions (2).
- Organ Transplant Rejection.
2. Contact Dermatitis.
What clinical features does Histamine cause?
- Pruritus.
- Vasodilation and Leakage - Hives, Angioedema, Hypotension.
- Smooth Muscle Contraction - Bronchospasm, Wheezing.
Investigations of Allergy (3).
- Skin Prick Testing.
- RAST (Radioallergosorbent Test). Testing.
- Food Challenge Testing.
Why are Skin Prick Tests and RAST Tests notoriously unreliable?
They assess sensitisation and not allergy.
Which investigation is the gold-standard?
Food Challenge testing - but requires lot of time and resources.
What is Skin Prick Testing? (4)
- A patch of skin is selected, usually on forearm.
- Select strategic allergen solutions e.g. peanuts, pollen and add a drop of each solution at marked points on the patch of skin with a water control and a histamine control.
- Use a fresh needle to make a tiny break at the site of each allergen drop.
- After 15 minutes, measure the size of the wheals.
What is Patch Testing? (2)
- A patch containing the allergen is placed on the patient’s skin.
- Assess skin reaction 2-3 days later.
What is Patch Testing most helpful in determining?
Allergic Contact Dermatitis in response to a specific allergen (not food allergies) e.g. latex, perfume, cosmetics, plants.
What is RAST Testing?
Measure total and allergen-specific IgE quantities in the patient’s blood sample.
What is the Food Challenge? (3)
- Start with almost non-existent quantities diluted further in other foods.
- Monitor kid very closely after exposure.
- Specialist Unit.
General Management of Allergy (3).
- Establish Correct Allergen, Avoid and Prevent.
- Prophylactic Antihistamines if Inevitable Contact.
- Adrenaline Auto-Injector if Risk of Anaphylaxis.
What is Immunotherapy?
Gradual exposure to allergens over months, with the aim of reducing reactions to certain foods/allergens.
