Hypersensitivity

Question 1

Define ‘hypersensitivity reaction’

Answer 1

Host response to an imagined insult or an over-reaction to a real (but sometimes unknown/unidentified) infectious agent.

Question 2

What are the types of hypersensitivity reaction?

Answer 2

1. Type I - annaphylactic, immediate
2. Type II - antibody dependant, cytotoxic
3. Type III - immune complex mediated
4. Type IV - cell mediated, delayed

Question 3

Define ‘auto-immune disease’

Answer 3

A specific humoural or cell mediated immune response against the constituents of the bodys own tissues (auto-antigens)

Question 4

Define a type 1 hypersensitivity reaction

Answer 4

• 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

Question 5

Define ‘anaphylaxis’

Answer 5

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)

Question 6

Give examples of type 1 hypersensitivity

Answer 6

• atopic dermatitis
• insect bite hypersensitivity (fleas, culucoides spp.)
• drug eruptions
• food allergies (not common in animals)

Question 7

Describe type IV hypersensitivity

Answer 7

• 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

Question 8

What type of hypersensitivity does the TB test trigger?

Answer 8

• Type IV

1. antigen introduced to SC tissue and processed by local APC
2. Th1 effector cells recognise the antigen and release cytokines which act on vascular endothelium
3. recruitment of T cells, phagocytes, fluid and protein to site of antigen injection causes a visible lesion

Question 9

Describe type III hypersensitivity

Answer 9

• 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