Type I hypersensitivity Flashcards
What are the important pieces of information to obtain from someone who has had an allergic reaction?
- Trigger
- Timing
- Symptoms
- Therapy
- Outcome
Why would someone not experience an allergic reaction when they are exposed to the allergen for the first time?
- Only become sensitized after the first exposure
Outline type I hypersensitivity
- Allergy
- Immunological basis for different diseases
- Immediate reaction <30 minutes
- Antigens (allergens) that are environmental and non-infectious
What causes a local type I hypersensitivity reaction?
- Ingested or inhaled allergen
- E.g. eczema, hayfever
What causes a systemic type I hypersensitivity reaction?
- Insect sting
- IV administration
- Ingestion
- Allergen directly in bloodstream
Give some examples of allergens that cause type I hypersensitivity reactions
- Seasonal exposure
- Perennial exposure
- Accidental exposure
What are some allergens that one may be exposed to seasonally?
- Tree and grass pollens
What are some allergens that one may be exposed to perennially?
- House dust mite
- Animal dander
- Fungal spores
What are some allergens that one may be exposed to accidentally?
- Insect venom
- Medicines (e.g. penicillin)
- Chemicals such as latex
- Foods: peanuts, milk, nuts, shellfish etc.
What are the mechanisms of type I hypersensitivity reactions?
- Abnormal adaptive immune response against an allergen (harmless substance)
- T helper 2 response
- IgE production
- Mast cell activation in sensitised individuals
Outline the hygiene hypothesis
- Children exposed to animals, pets and microbes in the early post natal period appear to be protected against certain allergic diseases
Outline the biodiversity hypothesis
- Western lifestyle induces alteration of the symbiotic relationships with parasites and bacteria leading to dysbiosis of the microbiome at mucosal surfaces
Outline the origin of mast cells
- Produced by bone marrow but only mature in tissue
- Once matured, mast cells express a receptor that binds IgE
- Also express a stem cell receptor important for differentiation and maturation
Where are mast cells located in the body?
- Most mucosal and epithelial tissues
- E.g. GI tract, skin, respiratory epithelium
- In connective tissue surrounding blood cells
What are the functions of mast cells?
- Recruit and activate effector cells
- Direct antimicrobial effects
- Physiological tissue remodelling
- Angiogenesis regulation
Give some examples of mast cell mediators
- Tryptase (enzyme)
- Histamine
- Leukotrienes
- Platelet activating factor
What are the biological effects of tryptase?
- Remodel connective tissue matrix
What are the biological effects of histamines?
- Toxic to parasites
- Increase vascular permeability
- Cause smooth muscle cell contraction
What are the biological effects of leukotrienes C4, D4, E4?
- Cause smooth muscle contraction
- Increase vascular permeability
- Stimulate mucus secretion
What are the biological effects of platelet-activating factor?
- Attracts leukocytes
- Amplifies production of lipid mediators
- Activates neutrophils, eosinophils, and platelets
What is the immune mechanism of the first allergen exposure?
- Direct activation of mast cells
- Leads to production of allergen specific IgE
What is the immune mechanism of the second allergen exposure?
- Allergen cross-links form
- Allergen binds to 2 different IgEs at cell surface of mast cells
- This activates mast cells and causes degranulation
What are some skin manifestations of allergic reaction?
- Urticaria
- Caused by mast cell activation within the epidermis
- Mediators are histamine and leukotrienes/cytokines
What are the facial manifestations of allergic reactions?
- Angioedema of the lips, eyes, tongue and upper respiratory airways
- Caused by mast cell activation in the deep dermis
- Mediated by histamine and bradykinin
