Immunology Flashcards
What is atopy?
A predisposition of having hypersensitivity reactions to allergens:
- eczema
- asthma
- allergic rhinitis
What is the importance of establishing and excluding allergies?
Food allergies can have a large psycho-social burden, particularly in those who have anaphylaxis.
It is not uncommon for symptoms and histories of ‘allergy’ to actually be a somatisation disorder rather than a true allergy. Allergy testing can play a role in reassuring patients they do not have a true allergy to certain foods.
What is the skin sensitisation theory of allergy?
There are two main contributors to a child developing an allergy to food:
- There is a break in the infant’s skin (ie. eczema, skin infection) that allows allergens from the environment to cross the skin and react with the immune system.
- 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, leading to sensitisation.
When a baby is weaned at around 6 months, if they are regularly eating foods that contain the allergen, their GI tract is regularly exposed to the protein. Therefore, regular exposure to an allergen through food and preventing exposure to the allergen through the skin barrier can help prevent food allergies developing.
What is Type 1 hypersensitivity reaction?
IgE antibodies react to an allergen, triggering mast cells and basophils to release histamines and cytokines.
This causes an immediate reaction.
What is a Type 2 hypersensitivity reaction?
IgG and IgM antibodies react to an allergen, activating the complement system and leading to direct damage to the local cells.
What is a Type 3 hypersensitivity reaction?
Immune complexes accumulate and cause damage to local tissues - often autoimmune.
What is a Type 4 hypersensitivity reaction?
Cell mediated hypersensitivity reactions caused by inappropriate activation of T-lymphocytes, leading to inflammation and damage to local tissues.
Give an example of hypersensitivity reactions:
a) Type 1
b) Type 2
c) Type 3
d) Type 4
a) food allergy reactions; anaphylaxis; asthma
b) haemolytic disease of the newborn; transfusion reactions
c) SLE; rheumatoid arthritis; HSP
d) organ transplant rejection; contact dermatitis
What are the main ways of testing for an allergy?
- skin prick testing (?sensitisation)
- RAST testing (?sensitisation)
- food challenge testing (?allergy)
What is the role of skin prick testing in allergy diagnosis?
Allergen solutions are selected and placed at marked points along the skin, alongside a water control and a histamine control.
A fresh needle is used to make a break in the skin at the site of each allergen.
At 15 minutes, the size of the wheals to each allergen are assessed and compared to the controls.
The issue with skin prick testing is it only tests for sensitisation; they often come back showing the patient is sensitised to many things you have tested for, and it doesn’t mean the patient has an allergy.
What is the role of RAST testing in allergy diagnosis?
Measures the total and allergen specific IgE quantities in the patients blood sample.
In patients with atopic conditions, the results will often come back positive for everything you test.
What is the role of food challenge testing in allergy diagnosis?
GOLD STANDARD
Food challenge is performed in a specialised unit with close monitoring. Patient is given increasing quantities of allergen to assess the reaction.
Management of allergy.
- establish correct allergen
- avoidance of allergen
- avoid foods that trigger reactions
- regular hoovering and changing sheets and pillows
- stay indoors when pollen count is high
- prophylactic antihistamines
- adrenaline auto-injector
In some cases, specialist centres may initiate immunotherapy. The patient is exposed to allergens over months, with the aim of reducing their reaction to certain foods or allergens.
Treatment of allergic reactions following exposure.
- antihistamines
- steroids
- intramuscular adrenaline
Antihistamines and adrenaline work by dampening the immune response to allergens.
What type of hypersensitivity reaction is anaphylaxis?
Type 1 hypersensitivity reaction - IgE stimulates mast cells to rapidly release histamine and other pro-inflammatory chemicals.
Presentation of anaphylaxis.
- urticaria
- itching
- angio-oedema
- abdominal pain
- shortness of breath
- wheeze
- stridor
- tachycardia
- collapse
Management of anaphylaxis.
ABCDE approach.
Intramuscular adrenalin - repeat after 5 mins if required.
Antihistamines.
Steroids (IV hydrocortisone).
Blood test to confirm anaphylaxis.
Mast cell tryptase within 6 hours of an anaphylactic reaction - this is released during degranulation and stays in the blood for 6 hours.
Indications for adrenalin auto-injectors.
Anaphylaxis.
Generalised allergic reactions with risk factors:
- asthma
- poor access to medical treatment (e.g. rural locations)
- adolescents
- nut or insect sting allergies
- significant co-morbidities
Prognosis of Cow’s milk protein allergy.
Most children will outgrow cow’s milk protein allergy by the age of 3.
Every 6 months, infants can be tried on the first step of the milk ladder and progress up the ladder until they develop symptoms. Over time, they should gradually be able to progress towards a normal diet containing milk.
