Allergy and anaphylaxisDr Sid Riddington

Allergy:What is allergy?Allergy is a immunologically mediated

hypersensitivity reaction.It is triggered by proteins in the environment

that would not affect a non—allergic patient. There are approximately 18,000,000 people

in the United Kingdom who suffer some form of allergy.

AllergyWhat forms of allergy are there?Grass pollen-hayfeverHouse dust mite-allegic rhinitis.Animal dander-dog and cat allergy.Proteins in foods-food allergyVenom allergy-reactions to wasp and bee

stings.Mould spores-occupational lung diseases.

Allergy:

Allergy:Allergies do tend to change with age:

Classically described as the allergic march.

Hayfever and rhinitis in childhood

Food allergies developing in adolescence.

These becoming quiescent in adult life, but

incidence of asthma increasing.

How do we get allergy:Initial exposure

Sensitization (with no initial adverse reaction)

Generation of antibodies (immunoglobulin E class)

Antibodies highly specific

Repeated exposure

Binding of antibodies to mast cells

Release of mediators from mast cells.

How do we get allergy:Mediators exert their effects to:Make the lining of airways swell

RhinitisConjunctivitisLaryngeal swelling

Make capillaries leak fluid to give oedemaAffect smooth muscle

AsthmaLow blood pressure

How do we get allergy:Allergy is a complex web of factors:Genetic:Family history is important.Small families with few children.Maternal diet and smoking.ObesityGenes linked to allergy found on

chromosomes 5 & 11

How do we get allergy:Allergy is a complex web of factors:

Environmental:

Smoking household in early years.

Early exposure to risky foods in diet.

Air pollution

Aerosols

Time of year of birth (just before the pollen season)

How do we get allergy:Protective factors:

Some viral illnesses (eg Hepatitis A and measles)

Living on a farm, especially one rearing livestock

Some types of intestinal microflora (Lactobacilli)

BCG vaccination

Heavy exposure to dog and cat allergens.

The cleanliness theory of allergy.

Allergy: Summary.Allergy is common and increasing.

Abnormal reaction to environmental chemicals

Potential allergens (things that can cause allergy

are legion)

Initial sensitisation phase followed by reexposure.

Complex and subtle web of factors including

genetic, environmental and immunological factors.

Venom allergyCan cause severe allergic reactionAbout 4 deaths per year in UK (coroner’s

data)Mostly caused by wasp stingsMost of population exposed to wasps.

Individuals get stung on average once every 10-15 years.

Smaller population exposed to bee stings – BKs & family

So most reactions in the larger but less stung population.

Venom allergyDifferent venoms in wasp and bee stings:

Bee sting allergens Phospolipase A2 and

Mellitin

Wasp sting allergens Antigen 5

Both contain enzymes called hyaluronidases

Sensitivity to both is very very rare.

Venom allergyNormal effect of either is

Localised redness (erythema)

Intense local pain

Local oedema and swelling.

A raised nettle-rash (urticaria) locally.

Venom allergyAllergic reactions can be local or generalLocal reactions affect only the area or limb of

the sting.Local reactions:More extensive swelling of the hand or limb.Can lead to blistering and secondary

infection.Not normally harmfulException: A “benign” local swelling in the

airway can be life-threatening.

Venom allergyGeneralised reactions:

These occur at sites remote from the sting.

THEY VARY WIDELY IN SEVERITY

The features of these include:

Redness and itching followed by

Hives (nettle rash) and facial or generalised swelling

Feeling of apprehension (Impending doom)

Venom allergyGeneralised reactionsBreathing difficulties because of

Swollen larynxAsthma

Low blood pressure giving symptoms ofLightheadednessGiddinessFainting or loss of consciousness

Venom allergyLess common features of generalised reactions

include

Abdominal pain

Incontinence

Central chest pain

Visual disturbances

Picture of someone having a generalised reaction is

very variable.

Venom allergyCourse of generalised reactions:Substantial proportion (20-80%) have no

general reaction to subsequent stings. Spontaneous improvement.

Some have much less severe general reactions.

Some have a variable pattern.Children do especially well- 95% have no

general reaction to subsequent sting.Course is not one of worsening reactions.

Venom allergyCauses for the unpredictability of reactions

Elapsed time

Immune response at time of sting

Dose of venom

Site of sting.

Venom allergyManaging local reactions

Antihistamines

Newer, rapid acting non-sedating type

Double dose on stinging.

Very large swellings may need intramuscular

antihistamines.

Venom allergySevere reactionsAdrenaline 300-500 micrograms IMNearly always effectiveDelay is fatal:Allows shock to developIncreases chance of treatment failurePatient may need antihistamines and steroids

later.

DiagnosisPositive history of being stung by

Skin prick testing or

Blood test for bee or wasp-specific antibodies

But the blood test gives a high incidence of

false positives.

Antibodies present but no history of allergy.

TreatmentDesensitisation

Probably only for those with severe general

reactions.

Time consuming and expensive

Can cause anaphylaxis.

Weekly injections for 3 months then

Monthly injections for 3 years.