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What is the difference between a hypersensitivity reaction and an autoimmune disorder?
A hypersensitivity reaction is an exaggerated or inappropriate immune response to a mild pathogen or innocuous substance (allergen) whereas an immune disorder is a specific hum oral or cell mediated immune response against the constituents of the body's own tissues (auto-antigens)
List the different types of hypersensitivity reactions
- Type I - anaphylactic, immediate
- Type II - antibody dependent, cytotoxic
- Type III - immune complex mediated
- Type IV - cell mediated, delayed
What type of antibody is produced during a type I hypersensitivity reaction?
What type of cell membrane is IgE bound to?
Mast cell membranes
What does exposure of antigens to IgE cause?
Mast cell degranulation which releases vasoactive mediators and causes a severe systemic reaction or milder local reaction
What type of vasoactive mediators are released from mast cells during degranulation?
Histamine and seratonin
What is anaphylaxis/anaphylactic shock?
An immediate systemic reaction caused by rapid, IgE mediated immune release of potent mediators from tissue mast cells and peripheral basophils
Give examples of things that can trigger anaphylaxis
- Foods eg peanuts
- But note up to 30% of cases are unidentified in humans
List examples of type I hypersensitivity in veterinary medicine
- Atopic dermatitis
- Insite bite sensitivity e.g. flea bite hypersensitivity or sweet itch
- Drug eruptions
- Food allergy (very rare)
What is the most common environmental allergen that causes atopic dermatitis?
House dust mites
What type of breeds are commonly predisposed to atopic dermatitis?
Westies, Cairns, Scotties, Boxers, etc
What are the presenting signs of atopic dermatitis and where is it commonly found on the body?
Pruritic dermatitis and otitis (conjuctivitis). On the face, feet and ventrum.
True or false: allergens that cause atopic dermatitis can only be inhaled?
False - it has not been established that allergens can be absorbed via the skin as well as being inhaled. Therefore abnormal skin barrier contributes to the pathogenesis of canine AD.
What is the presenting signs of flea bite hypersensitivity and where is it commonly found on the body?
Pruritic papular dermatitis. Dorsal lumbosacral area, neck, inner thighs and abdomen.
What is a type IV hypersensitivity reaction
These are delayed type hypersensitivity reactions. They involved the same processes as cell mediated immunity to microbial infection but the balance swings from protection to tissue damage if the stimulus is great or unusually persistent.
List the three types of Type IV hypersensitivity reactions
- Allergic contact dermatitis
Describe the tuberculin test
Antigen is introduced into subcutaneous tissue and processed by local antigen presenting cells. A T effector cell recognises antigen and releases cytokines which act of vascular endothelium. Recruitment of T cells, phagocytes, fluid and protein to site of antigen injection causes visible lesion.
Describe what happens during allergic contact dermatitis
The chemical binds to epidermal proteins to form an allergen