Type 1 Hypersensitivity Flashcards
What is Atopy?
A genetic tendency to produce IgE to normally innocuous environmental allergens
What is an allergy?
Clinical expression of atopic tendency
What is type 1 hypersensitivity mediates by?
Inappropriate production of specific IgE antibodies to harmless antigens
What do the antigens react with in type 1 hypersensitivity?
The IgE antibodies which are bound to mast cells
What happens when the antigen binds to the IgE bound to mast cells?
Degranulation of mast cells
What is sensitisation?
This is the initial event that lead to the specific IgE being developed for that allergen
What happens in sensitisation?
- CD4 cells recognise the allergen
* They proliferate and differentiate into T Helper 2 cells
What do the Th2 cells release?
IL-4, that stimulates the production of IgE by B Cells specific to that allergen
What does mast cells degranulation lead to the release of?
Mast cell mediators
What are the 2 forms of mast cell mediators?
Pre-formed
Newly synthesised
What are some examples of preformed mast cell mediators?
histamine
heparin
tryptase
What are some examples of newly synthesised mast cell mediators?
Prostaglandins
Leukotrienes
How are allergic reactions described?
Biphasic
What does biphasic mean?
Allergic reactions have an early phase and late phase reaction
What is responsible for the early phase reaction?
Histamine
What does histamine cause?
Vasodilation
Increased vascular permeability
Broncho-constriction
–symptoms of allergy
What is responsible for the late phase reaction?
TNF-α
What does TNF-α cause?
A localised inflammatory process at the site of exposure.
How long can the late phase take?
A few hours
What is anaphylaxis?
The most serious type of allergy
What happens in an anaphylaxis reaction?
vasodilation and increased vascular permeability.
Shift of fluids from the vascular to the extra-vascular space- fall in vascular tone.
Severe drop in blood pressure.
How does anaphylaxis present?
- Low blood pressure
- Angioedema
- Airway obstruction
What can cause anaphylaxis?
Foods - fish, shellfish, eggs, milk, wheat, nuts
Insect venoms - bee, wasp
Drugs - antibiotics, anaesthetic agents, antisera
What is the management of anaphylaxis?
Epinephrine (adrenaline)
How are allergies diagnosed?
History
Skin prick tests
Laboratory investigations: IgE levels (RAST test)
What are the possible managements of allergies?
Patient education
- Allergen avoidance - Antihistamines - local / systemic - Sodium cromoglycate - local / systemic - Steroid - local / systemic - Leukotriene antagonists - Desensitisation immunotherapy
What gene is associated with allergy?
Filaggrin gene
What is the hygiene hypothesis?
Increase in allergies in the developed world is caused by reduced exposure to microorganisms in early life.
What causes the symptoms in an allergic reaction?
Degranulation of mast cells and the release of mediators
When do mast cells initiate the allergic symptoms?
When the IgE and allergen interact
How are allergic responses different on repeat exposure?
Tend to get worse on repeat exposures due to increased sensitisation
