Define ‘hypersensitivity reaction’
Host response to an imagined insult or an over-reaction to a real (but sometimes unknown/unidentified) infectious agent.
What are the types of hypersensitivity reaction?
- Type I - annaphylactic, immediate
- Type II - antibody dependant, cytotoxic
- Type III - immune complex mediated
- Type IV - cell mediated, delayed
Define ‘auto-immune disease’
A specific humoural or cell mediated immune response against the constituents of the bodys own tissues (auto-antigens)
Define a type 1 hypersensitivity reaction
- Antigen specific IgE is produced
- IgE binds to mast cell membranes
- Re-exposure to the antigen causes mast cell degranulation
- Vasoactive mediators, histamine and PGs are released causing severe systemic reaction or a milder local reaction
Define ‘anaphylaxis’
An immediate systemic reaction caused by rapid IgE mediated immune release of potent mediators from tissue mast cells, and peripheral basophils. It involves the skin, +/- mucosal tissue (resp., GI, reduces BP)
Give examples of type 1 hypersensitivity
- atopic dermatitis
- insect bite hypersensitivity (fleas, culucoides spp.)
- drug eruptions
- food allergies (not common in animals)
Describe type IV hypersensitivity
- reactions involve the same processes as cell-mediated immunity to microbial infection - NOT dependent on antibody
- balance swings from protection to tissue damage if the stimulus is great or unusually persistent
- a sensitised T cell population develops after initial contact with antigen via APC
- tissue damage is due to activated macrophages and cytotoxic T cells
What type of hypersensitivity does the TB test trigger?
- Type IV
- antigen introduced to SC tissue and processed by local APC
- Th1 effector cells recognise the antigen and release cytokines which act on vascular endothelium
- recruitment of T cells, phagocytes, fluid and protein to site of antigen injection causes a visible lesion
Describe type III hypersensitivity
- direct damage mediated by antibodies to exposed cell surface antigens - antigen usually endogenous
- underlies various autoimmune disorders affecting various tissues e.g. haemolytic anaemia, pemphigus
