Hypersensitivities

Question 1

Describe Type I hypersensitivity

Answer 1

Phase 1/sensitisation phase:
- allergen exposure
- APCs capture antigen
- Naive T cell differentiation => cytokines released => B cell proliferation
- B cell class switch IgM => IgE
- IgE binds to mast cells which relocate to where allergen was encountered
- individual is sensitised

Phase 2:
- re-exposure
- primed IgE coated mast cells bind to antigen
- release of cytoplasmic granules e.g., histamine, heparin
- breakdown of mast cells
- pruritus, broncho-constriction, oedema
- immediate hypersensitivity

Question 2

Describe type II hypersensitivity

Answer 2

Antibody mediated cytotoxicity:
- IgG antibodies recognise ‘self’ antigens
- phagocytosis
- intrinsic ‘self’ antigens e.g., ion channels or hormone receptors are concerning

Question 3

How is type II hypersensitivity different to Type I?

Answer 3

Antibodies:
- IgG antibodies in type II
- IgE antibodies in type I
Location:
- Type II: antibodies in circulation
- Type I: antibodies bound to mast cells

Question 4

Give an example of type II hypersensitivity

Answer 4

Myasthenia gravis
- neuromuscular disorder
- auto-antibodies against neuromuscular junction

Question 5

Describe Type III hypersensitivity

Answer 5

Immune-complex hypersensitivity:
- formation of immune complexes
- deposited in tissues
- activation of complement system
- chemotactic for neutrophils
- neutrophils unable to ingest complexes in blood vessel walls
- release of toxic granules in tissue => severe tissue damage

Antibody excess
or Antigen excess => capillary deposition => vasculitis

Question 6

Give an example of type III hypersensitivity

Answer 6

Equine recurrent airway obstruction
- localised inflammatory reaction to mould within hay, dust etc

Canine blue eye
- excess antibody
- immune complexes in uveal tract
- inflammation and oedema in cornea

Question 7

Describe type IV hypersensitivity

Answer 7

T Cell mediated
‘delayed onset hypersensitivity’
Sensitisation phase:
- antigen specific T lymphocytes generated
Re-exposure:
- reactivation of sensitised cells
- travel to site
- release interferon gamma and other chemokines
- recruitment of macrophages, granulocytes etc. => inflammation

Question 8

Give examples of type I hypersensitivity

Answer 8

Anaphylaxis
Atopic dermatitis