Allergy Flashcards
What is the definition of hypersensitivity?
An immune response that causes collateral damage to self
What cell or antibody mediates Type I Hypersensitivity?
IgE
What cell or antibody mediates Type II Hypersensitivity?
IgG (mediates Cytotoxic hypersensitivity)
What cell or antibody mediates Type III Hypersensitivity?
Immune complex
antigen-antibody complex
What mediates Type IV Hypersensitivity?
T Cell-mediated
TH1 cells release cytokines to activate macrophages and Tc cells which cause direct cellular damage
How does IgE cause Type I Hypersensitivity?
- Antigen induces crosslinking of IgE bound to mast cells and basophils
- causes degranulation and releases vasoactive mediators
How does IgG cause Type II Hypersensitivity?
- Antibody is directed against antigens on cell
- cell is destructed by either complement activation or ADCC
What is ADCC?
Antibody-dependent cell-mediated cytotoxicity
How does the immune complex cause Type III Hypersensitivity?
- complex is deposited in various tissues
- complement activation
- massive neutrophil migration
Give an example of a typical manifestation of a Type I Hypersensitivity reaction?
- Anaphylaxis
- Local anaphylaxis
e. g. hay fever, asthma, eczema, hives, food allergy
Give an example of a typical manifestation of a Type II Hypersensitivity reaction?
- blood transfusion reaction
- autoimmune haemolytic anaemia
Give an example of a typical manifestation of a Type III Hypersensitivity reaction?
- Localised e.g. Arthus reaction
- Generalised e.g. necrotising vasculitis, rheumatoid arthritis, Systemic lupus erythematosus
Give an example of a typical manifestation of a Type IV Hypersensitivity reaction?
- Contact dermatitis
- Graft rejection
- Tubercular lesions
What causes an allergy?
An allergen
What is the “hygiene hypothesis”?
The thought that developing countries have less allergies as they are more exposed to an allergic environment.
Developed countries have higher levels of antibiotic use and better sanitation => develop more allergies.
How long after exposure does a Type I reaction occur?
Immediate reaction - occurs within minutes and up to 2 hours after exposure to allergen
What routes of exposure can provoke a Type I reaction?
skin contact
inhalation
ingestion
injection
Will a patient react the same way with every exposure to the allergen that provoked a Type I reaction?
Yes
reaction is consistent with every exposure
Give examples of Type I allergens which can be ingested
Nuts
Seafood
Give examples of Type I allergens which can be inhaled
Pollen
House dust mite
Give examples of Type I allergens which come into contact with the skin
Latex
Animal Dander
Give examples of Type I allergens which can be injected
Bee/wasp sting
Medication
How does Urticaria usually present clinically?
- Very itchy
- Lesions appear within 1 hour
- Lasts 2-6 hours, sometimes 24 hours
“hives”, “wheals”, “nettle rash”
How does angioedema usually present clinically?
- Localised swelling of subcutaneous tissue or mucous membranes
- Non pitting oedema
- Not itchy
What investigations can be done in the case of a Type I reaction?
- Specific IgE
- Skin prick or prick-prick testing
- Challenge test (only if SPT negative)
- Serum mast cell tryptase level (during anaphylaxis)
What is the risk of anaphylaxis during skin prick tests?
1:3000
What medication can be used to manage Type I allergic reactions?
- Anti-histamines
Prevent effects of mast cell activation - Corticosteroids
Anti-inflammatory - Adrenaline auto injector (for anaphylaxis)
- Sodium Cromoglycate
mast cell stabilisers => block mast cell activation
Aside from medication, what other forms of management are there for Type I allergic reactions?
- Allergen avoidance
- Immunotherapy
- Medic Alert bracelet
- Information and Education
What strength of adrenaline auto-injector should be prescribed for adults and for children?
300µg adults
150µg children
How long after exposure does a Type IV reaction occur?
24-48 hrs
Give examples of Type IV allergens which come into direct contact with the skin
preservatives in cosmetics
rubber accelerators
Give examples of Type IV allergens which are responsible for airbourne contact
fragrances
plants
Give examples of a Type IV allergen which is injected
Tattoo
What are the chambers on patch testing called and where are they applied?
Finn chambers
applied on the back
When are the patch tests removed and when are the readings taken?
Removed after 48 hours
Readings at 48 and 96 hours
Other than allergy, what can be the cause of dermatitis?
Endogenous
Irritant
What is irritant contact dermatitis?
- Non-immunological process
- Pattern depends on exposure
- Contact with agents that irritate skin directly
Give an example of an endogenous form of dermatitis?
Atopic eczema
- Dry skin and flexural
- Often associated with asthma and hayfever
