Allergy Flashcards
What are type I hypersensitivity reactions?
IgE mediated reactions in response to allergens. They occur within minutes and will cause mast cell degranulation and eosinophi activation.
What are type II hypersensitivity reactions?
This is an autoimmune cytotoxic reaction between cell bound antigen and IgM or IgG antibodies. Will develop after the antibody is prodcued and will remain as long as the antigen remains. An example is autoimmune haemolytic anaemia
What are type III hypersensitivity reactions?
It is an immune-complex reaction between circulating soluable antigens and IgG antibodys
What is a type IV hypersensitvity reaction?
Cellular immune response by lymphocytes, reaction occurs greater than 4 hours after exposure. e.g. topical dermatitis
Where are the IgE producing plasma cells for type I reactions mainly situated?
In the mucosal lympoid tissues adjacent to the respiratory and GI tracts
What are the stages of a type I hypersensitivity reaction?
Exposure to the allergen
Sensitisation and IgE production
Re-eposure to the allergen producing the response
What investigations can be done in allergy?
Serum specific IgE antibodies can be measured (RAST) this can be used to confirm a history
Skin prick testing for specific allergens however has a high false positive
Total IgE levels
What effects does histamine release have on the body?
Increased blood flow
Increased vascular permeability
Contraction of extravascular smooth muscle
Stimulation of nerves causing itching
What can be measured in the blood that is important for Diagnosing anaphylactic shock?
Tryptase levels rise in the blood after massive mast cell degranulation
How do eosinophils damage tissues in allergy?
They are attracted by Il-5 and when they bind to cross linked immunocomplex this causes release of cytotoxic proteins that damage tissues
What is the treatment for anaphylaxis?
500micrograms of adrenaline (0.5mls of 1/1000 adrenaline) This can be given every 10-15mins if needed
Maintain airway
Hydrocortisone can be given to help prevent the biphasic reaction
What is oral allergy syndrome?
Can be mistaken for food allergy as there is swelling but it is not systemic. This causes IgE antibodies in the mouth to react to food pollens, but it is only local
Why are drug reactions typically not type I hypersensitivity?
Drug molecules are generally too small to act as whole antigens
What is urticaria?
Eruption of migrating itchy wheals that occur as a result of histmine activation. Angiooedema is the deeper version of this and they can often occur together. Urticaria can occur spontaneously and is not always an allergy
What is the treatment for chronic spontaneous urticaria?
This is usually through strong antihistamines but in more serious cases can be cyclosporine and anti-IgE therapy
What are atopic conditions and give some examples?
More chronic conditions associated with destruction of tissues, involve eosinophils, TH2 cells and mast cells
Extrinsic allergic asthma, hayfever and eczema
What type of asthma are most childhood asthmas?
They are mostly extrinsic allergic asthmas, can be caused by dust mites, animals and pollens
What causes the biphasic response in allergic asthma?
Initial response due to histamine release and other pre formed mediators in respiratory mucosa. Later the second response is due to lymphocytes and granulocytes infiltrating and causing further inflammatory mediator release
Why do people with hay fever (atopic rhinitis) have greater risk of bacerial infections?
The oedema can cause blockage of the sinuses and eustatian tubes hence predisposing to secondary infection
What is the treatment for hayfever?
Using potent H1 receptor antagonists
Leukotriene receptor antagonists and anticholinergics can also be given in severe cases
What tests can be used to confirm allergic eczema?
Serum IgE is often raised, positive skin prick test to various antigens. Thought to be caused by T cells and S. aureus super antigens.
What are some examples of delayed hypersensitivity reactions (type IV)?
Contact dermatits
Coeliac disease
Sarcoidosis
Granulomatous TB
What is contact dermatitis?
Delayed hypersesitivity reaction that superficially resembles atopic eczema but is due to cell mediated inflammatory condition. Can be caused by nickel or copper
What sort of allergy is coeliac?
There are IgA antibodies specific to gluten
It is diagnosed through an initiaal screen of transglutaminase antibodies, transglutaminase enzymes are present in healthy individuals for breakdown of gluten
