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Hypersensitivity
Type I
- Allergic reactions
- Crosslinking of IgE leads to de-granulation of mast cells
- fast reaction - within 15-20 mins
Type II
- hypersensitivity triggered by binding of antigen to circulating blood
cells
- IgG binds antigen and leads to clearance of cell from blood stream -
predominantly by spleen
- leads to haemolytic anaemia or thrombocytopenia
Type III
- soluble antigens
- antigen:antibody immune complexes form
- usually IgG
- immune complexes deposit in blood vessel wall
- bind Fc receptors of leucocytes & activate complement leading to
immune activation and tissue destruction
- Local reaction is known as "Arthus reaction"
- immune complexes deposited in lung due to inhaled antigen - e.g. farmers
lung
Type IV
- delayed type hypersensitivity
- mediated by specific T-cells
- e.g. tuberculin test
- contact hypersensitivity
- can involve CD8 cells
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