Type I Hypersensitivity

Question 1

What is a type I hypersensitivity reaction?

Answer 1

Allergies
Can be a local or systemic reaction

Caused by allergens - environmental or non-infectious (proteins)

Question 2

What are some examples of allergens?

Answer 2

Seasonal:
Tree and grass pollen

Perennial exposure:
House dust mites, animal dander

Accidental exposure:
Insect venom, medicine, latex, food

Question 3

What are the mechanisms of type I hypersensitivity?

Answer 3

Abnormal adaptive immune response against allergen:
T helper cell response; IL-4, IL-5, IL-13
IgE production

Mast cell activation:
Different response depending on mast cell location
Occurs in sensitised individuals

Question 4

Where are mast cells located?

Answer 4

Most mucosal and epithelial tissues; GI tract, skin, resp

Also in connective tissue

Question 5

What are some mast cell mediators?

Answer 5

Tryptase
Histamine
Leukotrienes C4, D4, E4
Platelet-activating factor

Question 6

What is the role of tryptase in mast cell mediating?

Answer 6

Remodelling of connective tissue

Question 7

What is the role of histamine in mast cell mediating?

Answer 7

Increases vascular permeability

Smooth muscle contraction

Question 8

What is the role of leukotrienes in mast cell mediation?

Answer 8

Smooth muscle contraction
Inc vascular permeability
Stimulates mucus secretion

Question 9

What is the role of platelet-activating factor in mast cell mediating?

Answer 9

Attracts leukocytes

Activates neutrophils, eosinophils and platelets

Question 10

What antibody is produced in a type I hypersensitivity reaction?

Answer 10

IgE

Question 11

What occurs in mast cell degranulation?

Answer 11

Release of granule contents;
Histamine, chemokines

Synthesis of new mediators;
Leukotrienes, prostaglandins

Results in;
Inc vascular permeability, vasodilation, bronchial constriction

Question 12

What skin manifestation results from activation of mast cells within the epidermis?

Answer 12

Urticaria

Mediators: histamine, leukotrienes

Occurs within 24 hrs

Question 13

What is a manifestation of allergic reaction in the face?

Answer 13

Angioedema - mast cell activation in deep dermis

Mediators: histamine and bradykinin

Question 14

What can result from systemic activation of mast cells?

Answer 14

Hypotension
Cardiovascular collapse 
Generalised urticaria 
Angioedema
Breathing problems 

Anaphylaxis

Question 15

How is anaphylaxis treated?

Answer 15

IM adrenaline

α1 agonist - reverses peripheral vasodilation, alleviates hypotension
β2 agonist - reverses bronchoconstriction
β1 agonist - inc inotropy
Inhibits mast cell activation

Question 16

What is the definition of desensitisation?

Answer 16

Administration of increasing doses/allergen extracts over a period of years

Given to patients by injection or drops/tablets sublingually

Question 17

What are some drugs used to prevent mast cell activation?

Answer 17

Anti-histamine
Leukotriene receptor antagonists
Corticosteroids