Immunology

Question 1

What is an allergy?

Answer 1

A hypersensitivity of the immune system to allergens

Question 2

What is an allergen?

Answer 2

Any substance - most commonly proteins - which stimulate the production of IgE or a cellular immune response.

Question 3

What is atopy?

Answer 3

Predisposition to hypersensitivity to allergens - eczema, asthma, hayfever, allergic rhinitis and food allergies. This frequently runs in families.

Question 4

What is the mechanism of a Type 1 hypersensitivity reaction?

Answer 4

T helper cells respond to allergen and produce IgE antibodies. There is mast cell activation which causes release of histamines and cytokines to create an immediate reaction.

Question 5

What are the signs and symptoms of Type 1 Hypersensitivity reaction?

Answer 5

Symptoms begin within 30 minutes.
- hypotension
- urticarial rash (due to increased vascular permeability)
- angioedema - facial swelling
- breathing problems - due to bronchial constriction (anaphylaxis)

Question 6

What is the mechanism of a Type 2 hypersensitivity reaction?

Answer 6

IgG or IgM antibodies target cell bound antigens. This activates the complement reaction leading to tissue and cell damage (cytotoxicity)

Question 7

How quickly do symptoms of a type 2 hypersensitivity reaction occur?

Answer 7

5-12 hrs

Question 8

Give examples of type 2 hypersensitivity reactions

Answer 8

• haemolytic disease of the newborn
• transfusion reactions
• autoimmune haemolytic anaemia

Question 9

What is the mechanism of a Type 3 hypersensitivity reaction?

Answer 9

IgG or IgM immune complexes accumulate and deposit in local tissues causing damage.

Question 10

How quickly do type 3 sensitivity symptoms occur?

Answer 10

3-8 hours

Question 11

What are examples of Type 3 Hypersensitivity reactions?

Answer 11

• SLE - systemic lupus erythematosus
• rheumatoid arthritis
• infectious (post streptococcal) glomerulonephritis
• Henoch-Schonlein purpura

Question 12

What is the mechanism of a Type 4 hypersensitivity reaction?

Answer 12

Cell mediated hypersensitivity caused by T lymphocyte activation causing inflammation and damage to local tissues

Question 13

What are some examples of type 4 hypersensitivity reactions?

Answer 13

• contact dermatitis
• organ transplant rejection
• granulomatous conditions - e.g. TB, Sarcoidosis, Leprosy

Question 14

How is an allergy diagnosed?

Answer 14

• detailed history
• skin prick testing - only tests sensitivity not allergy
• RAST testing (blood tests for total and specific IgE)
• food challenge test - gold standard

Question 15

What should be asked in an allergy history?

Answer 15

• timing after exposure
• previous exposure to allergen
• symptoms - e.g. rash, swellings, wheeze
• personal and family history of atopy and allergies

Question 16

How should allergies be managed?

Answer 16

• avoidance of the allergen where possible
• prophylactic antihistamines - e.g. for hay fever and allergic rhinitis
• give epi-pen to those at risk of anaphylaxis

Specialist centres may initiate immunotherapy - gradual exposure to the allergen

Question 17

How should allergic reactions be managed (after exposure)?

Answer 17

• antihistamines - cetirizine
• steroids - e.g. oral pred or IV hydrocortisone
• IM adrenaline in anaphylaxis

Question 17

How should allergic reactions be managed (after exposure)?

Answer 17

• antihistamines - cetirizine
• steroids - e.g. oral pred or IV hydrocortisone
• IM adrenaline in anaphylaxis