Immunology - Allergy

Question 1

What are allergens?

Answer 1

Antigens that the immune system recognises as foreign and potentially harmful leading to an allergic immune response

Question 2

What is atopy?

Answer 2

Predisposition to hypersensitivity reactions to allergens

Question 3

What is the skin sensitisation theory of allergy?

Answer 3

Break in infants skin e.g. eczema or infection, allows allergens such as peanut proteins from environment to cross skin and react with immune system

Child has no contact with allergen from GI tract, absence of GI exposure to allergen

Allergens entering through skin recognised by immune system as being harmful and foregin

Immune system becomes sensitised, next encounter causes full immune response

When baby is weaned, GI tract can be exposed to proteins, the GI tract recognises allergen as food and not a foreign harmful protein, immune system no longer responds to it

Overall, theory is that regular exposure to allergen through food and preventing exposure through skin prevents allergy development

Question 4

What classification is used for hypersensitivity reactions?

Answer 4

Coombs and Gell classification

Question 5

What is a type 1 hypersensitivity reaction?

Answer 5

Type 1
IgE antibodies to allergen trigger mast cells and basophils

Histamine and other cytokines released

Immediate reaction

Food allergies

Question 6

What is a type 2 hypersensitivity reaction?

Answer 6

Type 2
IgG and IgM antibodies react to allergen

Complement system activated

Direct damage to local cells

Haemolytic disease of new-born
Transfusion reactions

Question 7

What is a type 3 hypersensitivity reaction?

Answer 7

Type 3
Immune complexes accumulate and cause local tissue damage

SLE
Rheumatoid arthritis
Henoch-Schoenlein purpura

Question 8

What is a type 4 hypersensitivity reaction?

Answer 8

Type 4
Cell mediated hypersensitivity reactions caused by T lymphocytes

T cells inappropriately activated

Inflammation and damage to local tissues

Organ transplant rejection
Contact dermatitis

Question 9

What important parts of a history must be covered?

Answer 9

• Timing after allergen exposure
• Previous and subsequent exposure and reaction to allergen
• Symptoms of rash, swelling, breathing difficulty, wheeze and cough
• Previous personal and family history or atopic conditions and allergies

Question 10

What investigations are used to determine allergy?

Answer 10

History

3 main ways to test for allergy
- Skin prick testing
- RAST testing
- Food challenge testing

Question 11

What do skin prick testing and RAST testing assess?

Answer 11

Sensitisation
Not allergy

Often come back showing patient is sensitised to many things tested for, very challenging to explain to child and parents that a positive test does not mean unsafe to eat those foods

Question 12

What is the gold standard allergy investigation?

Answer 12

Food challenge testing

Question 13

What is the downside of food challenge testing?

Answer 13

Requires lots of time and resources

Only available in select areas

Question 14

What is skin prick testing?

Answer 14

Patch of skin in on forearm selected

Allergen solutions selected e.g. peanuts, house dust mite and pollen

Drop of allergen solution placed at marked points along patch of skin with water control + Histamine control

Fresh needle used to make tiny break in skin at each allergen site

After 15 minutes, size of wheals to each allergen are assessed and compared to controls

Question 15

What is patch testing most helpful for diagnosing?

Answer 15

Allergic contact dermatitis in response to a specific allergen

Question 16

What are some examples of allergic contact dermatitis?

Answer 16

Latex
Perfumes
Cosmetics
Plants

Question 17

How is patch testing done?

Answer 17

Patch containing allergen placed on skin

Patch either contains a specific allergen or a grid of lots of allergens as a screening tool

2-3 days later skin reaction assessed

Question 18

What is RAST testing?

Answer 18

Measures total and allergen specific IgE quantities in blood

Patients with atopic conditions e.g. eczema, results come back positive for nearly everything

Question 19

How is food challenge done?

Answer 19

Specialist unit with very close monitoring

Child given increasing quantities of allergen to assess reaction

Start with almost nothing diluted further in other foods e.g. small amount of peanut in a chocolate bar

Children monitored very closely after each exposure

Helpful excluding allergies for reassurance

Question 20

How are allergies managed?

Answer 20

• Establish correct allergen
• Avoid allergen
• Avoid foods that treigger reactions
• Regular vacuuming and sheet changes when allergic to house dust mites
• Stay inside with high pollen count
• Prophylactic antihistamines when contact is inevitable e.g. hay fever
• Adrenaline auto-injectors for high risk anaphylactic reaction patients

Question 21

What is immunotherapy?

Answer 21

Specialist centres gradually expose patients to allergens over months

Aims to reduce reaction to certain foods or allergens

Question 22

After exposure how should allergic reactions be treated?

Answer 22

Anithistamines - e.g. cetirizine
Steroids - oral pred, topical or IV hydrocortisone
IM adrenaline in anaphylaxis

Close monitoring to ensure not anaphylaxis progression

Question 23

How do antihistamines and steroids work generally?

Answer 23

Dampen immune response to allergens