5- Allergy Flashcards
define allergy
- Hypersensitivity’s of the immune system to allergens
- Atopy is a term which describes a predisposition to having hypersensitise reactions to allergens
pathophysiology of allergy
Allergens are antigens (proteins) which the immune system recognises as foreign and potentially harmful, leading to an allergic response
Causes/risk factors for allergy
- Atopy
o Eczema
o Asthma
o Hayfever
o Allergic rhinitis
o Food allergies - Genetic
investigating allergy
3 methods
- skin prick testing
- RAST testing
- Food challenge testing (gold standard)
skin prick testing
- Assess’ sensitisation and not allergy (can be misleading- can often show many sensitisation and hard to explain to parent that their child can still safely eat foods)
- Protocol: patch of skin selected e.g. forearm. Allergen solutions selected e.g. peanuts, house dust mite and pollen. A drop of each allergen placed at marked points along patch of skin, along with a water control and histamine control. A fresh needle is used to make a tiny break in the skin at each site. After 15 mins, size of the wheals to each allergen are assessed and compared to the controls
RAST testing
- Assess’ sensitisation and not allergy
- Protocol: blood tests which measures total and allergen specific IgE quantities in patients blood sample. If patient atopic- often positive results for everything
Food challenge testing
Gold standard- tests allergy
often used if skin prick testing suggests allergy to confirm
- Protocol: performed in specialised unit with very close monitoring
- Starts with almost non-existent quantities diluted further in other foods e.g. mixing a tiny amount of peanuts into a bar of chocolate
- Children monitored very closely after each exposure
management of allergy
- Avoidance of established allergen
- Regular hoovering and changing of sheets and pillows in patients allergic to dust mites
- Stay indoors when pollen count high
- Prophylaxis antihistamines when contact is inevitable e.g. hayfever
- Epipen if at risk of anaphylaxis
- Immunotherapy in specialist centres: involves exposing patient to allergens over months
importance of excluding allergies
- Huge psychosocial burden if diagnosed
o Risk of anaphylaxis
o Responsibility of avoiding allergens and always carrying epipen - Can be a somatisation disorder rather than true allergy
o If wrongly diagnosed as an allergy can lead to restrictive or unhealthy eating
orign of allergies theory: skin sensitisation theory of allegry
The skin sensitisation theory is currently the leading theory on the origin of allergies. This theory suggests there are two main contributors to a child developing an allergy to a food:
1. There is a break in the infant’s skin (from eczema or a skin infection) that allows allergens, such as peanut proteins, from the environment to cross the skin and react with the immune system.
2. The child does not have contact with that allergen from the gastrointestinal tract, and there is an absence of GI exposure to the allergen.
The theory is that allergens entering through the skin are recognised by the immune system as being foreign and harmful proteins. The immune system reacts by becoming sensitised to that allergen, so that when it next encounters that allergen again it will launch a full immune response (an allergic reaction).
When a baby is weaned at around 6 months, if they are regularly eating foods that contain that allergen, their GI tract is regularly being exposed to that protein. The GI tract will recognise that allergen as a food and not a foreign or harmful protein, and inform the immune system that it is a safe thing to be exposed to.
The theory is that regular exposure to an allergen through food and preventing exposure to that allergen through the skin barrier can help prevent food allergies developing.
definitions
hypersensitivity conditions
Conditions
* Asthma
* Atopic eczema
* Allergic rhinitis
* Hayfever
* Food allergies
* Animal allergies
Hypersensitivity: Coombs and Gell classification
- Type 1: IgE antibodies to a specific allergen trigger mast cells and basophils to release histamines and other cytokines. This causes an immediate reaction. Typical food allergy reactions, where exposure to the allergen leads to an acute reaction, range from itching, facial swelling and urticaria to anaphylaxis.
- Type 2: IgG and IgM antibodies react to an allergen and activate the complement system, leading to direct damage to the local cells. Examples are haemolytic disease of the newborn and transfuse on reactions.
- Type 3: Immune complexes accumulate and cause damage to local tissues. Examples are autoimmune conditions such as systemic lupus erythematosus (SLE), rheumatoid arthritis
- Type 4: Cell mediated hypersensitivity reactions caused by T lymphocytes. T-cells are inappropriately activated, causing inflammation and damage to local tissues. Examples are organ transplant rejection and contact dermatitis.
Evaluating a child with a suspected food allergy
1) Medical history
2) Physical examiantion
3) Screening tests
- skin prick tests
- blood specific IgE
4) Diagnosis verification with controlled oral food challenge
5) Interpretation of tests
medical history
physical examination
Screening tests
IgE mediated food allergy
- Skin prick tests
- blood specific IgE
Non-IgE
- elimination diet
diagnosis verification
allergy tests detect
sensitisation and not allergy
BUT
- patient often translate their result into clinical allergy
test are good at confirming…
Non allergy
- the higher the IgE the higher the Positive predicitve value
dose of intramuscular adrenaline for anaphylaxis
management of cows milk protein allergy
1) Avoiding cows milk
- Breast feeding mothers should avoid dairy
- Replace formula with special hydrolysed formulas of cows milk allergy -> contain broken down proteins which don’t trigger immune response
- Replace formula with amino acid formula- more expensive, good for children with severe allergys
-
2) Every 6 months infants can be tried on the first step of the milk ladder e.g. malted milk biscuits and slowly progress up it until dthey develop symptoms
presentation of urticaria
- Pruritus
- Vascular permeability
- Oedema
- Rash: erythematous swelling of various shapes and sizes, classically have central pallor with erythematous flare
