Allergy & The Skin

Question 1

What mediates type 1 hypersensitivity/allergy

Answer 1

IgE

Question 2

When does a type 1 hypersensitivity/allergy present with respect to allergen exposure

Answer 2

Immediate reaction - occurs within minutes & up to 2 hours after exposure to allergen

Question 3

What route of exposure causes type 1 hypersensitivity/allergy

Answer 3

Skin contact e.g. latex
Inhalation e.g. pollen, house dust mite
Ingestion e.g. nuts, seafood
Injection e.g. bee sting, medications

Question 4

Describe the pathophysiology of type 1 allergy

Answer 4

• Sensation stage: production of specific IgE by B cells (helped by T cells) in response to initial allergen exposure, residual IgE antibodies bind to circulating mast cells via Fc receptors
• Allergic stage: on re-exposure to allergen, the allergen will bind to IgE coated mast cells → cell degranulation (release of histamine and other inflammatory mediators)

Question 5

Type 1 allergy clinical features

Answer 5

• Urticaria (wheels & hives)
• Angioedema (localised, non pitting oedema)
• GI symptoms (N&V, abdominal pain, diarrhoea)
• Respiratory (wheezing, sneezing, nasal itching, rhinorrhoea)
• Anaphylaxis

Question 6

Oedema in allergy vs swelling secondary to atopic dermatitis - how do you tell the difference

Answer 6

Atopic dermatitis is likely to be scaly, itchy, weeping etc with less pronounced swelling
Allergic oedema is likely to be accompanied by hives & other allergic symptoms

Question 7

Type 1 Allergy investigations

Answer 7

1. History
2. Specific IgE (RAST) blood test
3. Skin prick test (if IgE negative)
4. Challenge test (only if SPT is negative)

Question 8

Anaphylaxis investigation

Answer 8

Serum mast cell tryptase level (during anaphylaxis)

Question 9

Type 1 allergy management

Answer 9

• Allergy avoidance
• Anti histamines (prevents effects of mast cells) (1st line)
• Corticosteroids (anti inflammatory agent) (2nd line)
• Adrenaline auto injector (for anaphylaxis)
• Sodium cromoglicate (mast cell stabiliser/blocker)

Question 10

When does a type IV hypersensitivity/allergy present with respect to allergen exposure

Answer 10

Delayed hypersensitivity - Onset of reaction typically after 12-24 hours

Question 11

What mediates Type IV allergy

Answer 11

T Cell mediated, antigen specific

Question 12

What is type IV allergy aka

Answer 12

Allergic contact dermatitis

Question 13

What route of exposure causes type IV hypersensitivity/allergy

Answer 13

Direct skin contact e.g. cosmetic preservative, nickel in belt
Airborne contact e.g. fragrances, plants
Injection e.g. tattoo

Question 14

Type IV Allergy investigations

Answer 14

Patch testing

Question 15

What can be mistaken for allergic contact dermatitis

Answer 15

Irritant contact dermatitis
Endogenous dermatitis e.g. eczema
Infection

Question 16

Irritant/ Allergic contact dermatitis management

Answer 16

•Allergen / Irritant avoidance
•Allergen / Irritant minimisation
•Emollients
•Topical steroids
•UV phototherapy
•Immunosuppressants

Question 17

Type IV allergy/ allergic contact dermatitis pathophysiology

Answer 17

1. Sensation stage: generation of memory T cells following exposure to antigen (via Langerhans cells in epidermis and MHC-II)
2. Allergic stage: activation of sensitised Th cells in response to antigen, causing release of inflammatory cytokines and cell-mediated cytotoxicity

Question 18

Allergic contact dermatitis clinical features

Answer 18

itchy, eczematous rash (vesicles, fissures, erythema)

Question 19

Allergic vs irritant contact dermatitis clinical features

Answer 19

• In allergic contact dermatitis the rash may extend beyond the boundaries of immediate contact
• This is unlike irritant contact dermatitis, where the skin changes are localised directly to the area of exposure
• Allergic contact dermatitis requires a sensitisation phase whereas irritant contact dermatitis doesn’t

Question 20

Common irritants

Answer 20

Detergents, bleach, PPD, nappy rash (urine - flexures will be spared), ‘lip-lick’ chelitis