Lecture 7.2 - Allergies Flashcards
What is an allergen?
A non-infectious antigen
Briefly explain how we believe allergies develop?
At birth the Th2 phenotype is dominant. Upon exposure to allergens and microbes, this should switch to Th1. However, if this exposure does not occur, the Th2 phenotype persists. Part of this phenotype is the activation of eosinophils and IgE, both involved in allergy
What is “atopy”?
The collective term for asthma, eczema and hayfever
What is the key cell involved in allergic responses?
Mast cell
Describe the structure of a mast cell
Contains granules containing chemical mediators such as histamine, leukotrienes and prostaglandins
Where are mast cells located and why is this significant in allergies?
Located on all mucosal surfaces, hence close to blood vessels. Therefore mediators can quickly enter bloodstream after degranulation
What are the 2 phases of all hypersensitivity reactions?
Sensitisation phase
Activation/effector phase
Briefly outline what occurs during the sensitisation phase of a hypersensitivity reaction
Occurs on the first encounter with an antigen
Antigen-specific IgE is produced, which binds to the mast cells. They are then armed against this antigen.
Phase is clinically silent
Briefly outline what happens during the activation phase of a hypersensitivity reaction
Occurs on re-exposure to a particular antigen
The specific IgE for this antigen (already bound to the mast cells) is cross-linked
This activates the mast cells –> degranulation –> release of chemical mediators
What 3 major effects does degranulation have on the body?
Increased vascular permeability
Vasodilation
Bronchoconstriction
Give some examples of symptoms of a localised allergic reaction
Skin - wheal and flare
Face - angioedema of lips, mouth, tongue and eyes
What is anaphylaxis?
Systemic allergic reaction which occurs following systemic absorption of an allergen, either via the GI tract or IV
What are the symptoms of anaphylaxis?
Sudden onset and rapid progression of:
- Generalised urticaria and angioedema
- Severe hypotension
- Bronchoconstriction
What is the treatment for anaphylaxis?
What is its mechanism, in broad terms?
IM adrenaline
Reverses vasodilation and bronchoconstriction, and increases force of contraction of heart
Give examples of how you might diagnose an allergy
Clinical history
Serum level of allergen-specific IgE or chemical mediators
Skin prick tests
