allergies in children Flashcards
IgE mediate allergy - diagram
pathophysiology of IgE mediated allergy
- presentation of allergen to class II MHC molecule
- release of various interleukins and messenger compounds - direct effects on cells and influence B cells
- class switching and production of specific immunoglobulin E
- IgEs bind to mast cells - sensitisation to a particular allergen
- subsequent exposure to allergen - cross binding of 2 IgE molecules, degranulation, release of mediators from mast cells
- direct effects on epithelial cells, fibroblasts, smooth muscles and blood vessels
what is a class II MHC molecule
APC which binds to T helper cells
which interleukins released by T cells affect B cells
T2 helper cells via IL 4 and IL 13 affect the B cell
mast cell degranulation leads to
release of inflammatory mediators
mast cell degranulation - clinical relevance
rapid release and onset of symptoms:
- histamine, tryptase, hydrolase
later release w/ subsequent effects:
- secreted inflammatory mediators - prostaglandins, leukotrienes, platelet activating factors, cytokines
what does histamine induce
- bronchial smooth muscle contraction
- vasodilation → flushing, hypotension if sustained
- separation of endothelial cells → hives, subcutaneous oedema
- activation of nerve endings → pain and itching
what is the genetic influence to allergy
- paternal atopy (maternal)
- concordance for allergy in twins
- neither parent w/ atopy - 14% risk of atopy in child, one 30% and two 60%
- hygiene hypothesis
how to tell if it is an allergy
- rapid onset
- histamine mediated reactions
- urticaria, erythema, angioedema, pallor/sweating, wheeze
- improvement w/ antihistamines
- relatively quick resolution of symptoms
possible triggers for allergic reaction
food
environmental allergen - pollen, house dust mites etc
drug
sting/bite
idiopathic
common food allergens (>90%)
- milk
- hen’s egg
- peanut
- tree nuts
- soya
- wheat
- fish
- sesame
which allergies are more common in which countries
cow’s milk - 8x more common in UK and US than in Israel
peanut allergy - 0.2% in israel vs 2.2% in US, UK and canada
shellfish allergy more common in singapore than UK
determining severity of reaction
mild/mod: angioedema (not involving airway), urticaria and rash
severe: angioedema of airway (stridor), bronchospasm, peripheral vascular dilation → hypotension
supporting evidence to an episode being an allergy
previous reactions
atopy
Fhx - allergy, atopy (siblings and other first degree relatives)
response to treatment
co-existing asthma
investigations for allergies
skin prick testing
specific IgE
oral food challenge
why is skin prick testing first line investigation
easy to perform
non-invasive
immediate results (20 mins)
cheap
-ve SPT is an excellent predictor for a -ve IgE mediated food reaction in pts w/ anaphylaxis (>95%)
negatives of SPT
must stop antihistamines 48hrs prior - importance of +ve control
broken skin
theoretical risk of anaphylactic reactions
dermatographism - all results would be +ve, importance of -ve control
over-interpretation of +ve results - sensitisation and therefore false +ve response
avoid random tests
pros and cons to specific IgE testing
pros:
- no need to stop antihistamines
- no risk of reactions
cons:
- expensive and invasive
- delay in obtaining results
- less sensitive and specific than SPT
- highly unreliable results in eczema
strong +ve predictive values on specific IgE testing
- less clearly established in younger children
SPT vs specific IgE testing
oral food challenge
day case procedure
gold standard
exposure to allergen in controlled environment - tells you what actually happens upon contact or ingestion
- done in stepwise manner with close observation
making a diagnosis of allergy
clear hx
worst reaction - guides treatment options
supporting evidence from investigations
identify and advise on allergen avoidance
how common is urticaria and angioedema
lifetime prevalence of 8.8%
chronic in 30-45%
urticaria alone 50%
urticaria and angioedema 40% (up to 85%)
angioedema alone 10%
up to 20% of those referred to hospital remain symptomatic after 10yrs