Allergy and the skin

Question 1

Give the 4 types of hypersensitivity and examples

Answer 1

1- Ig E mediated, e.g hey fever, asthma, hives, eczema, food allergy

2- IgG cytotoxic hypersensitivity e.g blood transfusion

3- immune complex mediated e.g RA, SLE

4- T cell mediated e.g contact dermatitis & graft rejection

Question 2

Type 1 allergy

• history
• pathophysiology
• presentation (4)
• investigations
• management

Answer 2

-consistent, immediate reaction with each exposure

-allergen picked up by dendritic langerhan cell
passed on to T cells
T helper cells produce IL4,5 & 13
stimulate B cells to produce allergen specific IgE
IgE attaches to mast cells and they degranulate
produce histamines, cytokines, leukotrines, prostoglandins
cause itch/vomiting/cramping/diarrhoea

-Urticaria- very itchy, lesions appear within hours
Angiodema- localised swelling of subcutaneous tissues and mucous membranes, non pitting
wheezing/asthma
anaphylaxis- severe life threatening, A= pharyngeal/larangeal odema B= bronchospasm C= hypotension/tachycardia

-RAST, blood test specific to IgE
Skin prick testing
challenge test if SPT neg
serum mast cell tryptase level (during anaphylaxis)

-allergen avoidance
anti-histamines
Corticosteroids
adrenaline autoinjection
mast cells stabiliser- sodium chromoglycate
immunotherapy
\+medic alert bracelet

Question 3

Describe the process of a RAST test

Answer 3

“The suspected allergen is bound to an insoluble material and the patient’s serum is added. If the serum contains antibodies to the allergen, those antibodies will bind to the allergen. Radiolabeled anti-human IgE antibody is added where it binds to those IgE antibodies already bound to the insoluble material. The unbound anti-human IgE antibodies are washed away. The amount of radioactivity is proportional to the serum IgE for the allergen”

Question 4

Define non-allergic reactions?

Answer 4

those that are not mediated by IgE

Question 5

Type 4 hypersensitivity

• presentation
• allergens
• pathophysiology (2 phases)
• investigation
• management

Answer 5

-Allergic contact dermatitis, 24-48 hrs after contact
think about contact exposure, match allergen to position and distribution of dermatitis

-cosmetics, rubber preservatives, fragrances, plants, injection

-Sensitisation phase
haptens penetrate epidermis and taken up by epidermal cells and langerhans cells which migrate to nodes
there they present the haptenated peptides to CD8 and CD4 T cells, specific T cell precursors then clonal expand in the nodes and circulate in the blood and return to tissue
elicitation phase
process re-occurs and the dendrites and keratinocytes present happen to T cells activating cytotoxic T lymphocytes which cause apoptosis of keratinocytes & production of cytokines & chemokines
leukocytes are recruited to the skin

-patch testing with readings at 48 and 96 hrs

-allergen/irritant avoidance
emollients
topical steroids
Uv phototherapy 
immunosuppressants

Question 6

define irritant contact dermatitis?

Answer 6

non-immunological process
due to contact with agents that abrade, irritate and traumatise skin directly so don’t need prior sensitisation
pattern is dependent on exposure