Allergy Flashcards
What is allergy?
Allergy is an umbrella term for hypersensitivity of the immune system to allergens. Allergens are proteins that the immune system recognises as foreign and potential harmful, leading to an allergic immune response. These proteins are types of antigen. Antigens are proteins that can be recognised by the immune system. The body will come in contact with millions of different antigens, and very few will lead to a hypersensitivity reaction. The ones that do are called allergens.
What is atophy?
Atopy is a term used to describe a predisposition to having hypersensitivity reactions to allergens. It refers to the tendency to develop conditions such as eczema, asthma, hayfever, allergic rhinitis and food allergies. These conditions are referred to as atopic conditions. Patients often have more than one atopic condition, and atopy frequently runs in families.
Why is it important to establish and exclude allergies?
Having a food allergy can be a huge psycho-social burden, particularly in those who have anaphylaxis or an epipen. It means checking all food labels for ingredients, ensuring all those responsible for the child are aware (e.g. school, other parents and relatives) and being very cautious or avoiding eating out in restaurants or anywhere with unlabelled food, where allergens may have made their way into foods.
It is not uncommon for symptoms and histories of “allergy” to actually be a somatisation disorder rather than a true allergy. It is important to establish whether symptoms are down to an allergy, or more psychological, because an allergy diagnosis can lead to restrictive or unhealthy eating and do more harm than good. Allergy testing can play a role in reassuring patients that they do not have a true allergy to certain foods.
Briefly describe the ‘The Skin Sensitisation Theory of Allergy’
The Skin Sensitisation Theory of Allergy
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:
- 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.
- 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.
What conditions are due to hypersensitivity reactions?
- Asthma
- Atopic eczema
- Allergic rhinitis
- Hayfever
- Food allergies
- Animal allergies
What classification system is used to differentiate hypersensitivity reactions?
The Coombs and Gell classification is used to describe the underlying pathology of different hypersensitivity reactions.
Briefly describe ‘Type 1’ hypersensitivity reactions
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.
Briefly describe ‘ Type 2’ hypersensivity reactions
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 newbornand transfusion reactions.
Briefly describe ‘Type 3’ hypersensitivity reactions
Type 3: Immune complexes accumulate and cause damage to local tissues. Examples are autoimmune conditions such as systemic lupus erythematosus (SLE), rheumatoid arthritis and Henoch-Schönlein purpura (HSP).
Briefly describe ‘Type 4’ hypersensitivity reactions
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.
Briefly describe an allergy history taking
A detailed history is the most important aspect of diagnosing allergies. Allergies can be diagnosed on history alone and investigations are impossible to accurately interpret without a clear history. Important areas to cover include:
- Timing after exposure to the allergen
- Previous and subsequent exposure and reaction to the allergen
- Symptoms of rash, swelling, breathing difficulty, wheeze and cough
- Previous personal and family history of atopic conditions and allergies
What investigations can be used to test for allergy?
The most reliable information about whether a patient has an allergy is a clear and detailed history.
There are three main ways to test for allergy:
- Skin prick testing
- RAST testing, which involves blood tests for total and specific immunoglobulin E (IgE)
- Food challenge testing
What is the role of skin prick testing and RAST testing for allergy?
Skin prick testing and RAST testing assess sensitisation and not allergy. This is important, because it makes these tests notoriously unreliable and misleading.
Think carefully before performing an allergy test, particularly a RAST test. They often come back showing that the patient is sensitised to many of the things you have tested for, and it becomes very challenging to explain to the child or their parents that the positive test results do not mean it is unsafe for the child to eat those foods.
What is the role of food challenge testing for allergy?
Food challenge testing is the gold standard investigation for diagnosing allergy, however it requires a lot of time and resources and is only available in selected places.
Briefly describe the process of skin prick testing
A patch of skin is selected, usually on the patients forearm. Strategic allergen solutions are selected, for example peanuts, house dust mite and pollen. A drop of each allergen solution is placed at marked points along the patch of skin, along with a water control and a histamine control. A fresh needle is used to make a tiny break in the skin at the site of each allergen. After 15 minutes, the size of the wheals to each allergen are assessed and compared to the controls.
