14 - Allergy and Immunology Flashcards
What is the definition of:
- Allergy
- Atopy
Allergy: hypersensitivity of the immune system to antigens, can be IgE or non-IgE mediated
Atopy: predisposition to having hypersensitivity reactions e.g eczema, asthma, hay fever, allergic rhinitis, food allergies
What is the definition of
- Allergen
- Sensitisation
What is the skin sensitisation theory?
- There is a break in the infant’s skin (from eczema or a skin infection) that allows allergens, such as peanut proteins, to cross the skin and react with the immune system
- The child does not have contact with that allergen from the GI 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)
What are some hypersensitivity medical conditions?
Allergic diseases occur when individuals make an abnormal immune response to harmless environmental stimuli
- Asthma
- Atopic eczema
- Allergic rhinitis
- Hayfever
- Food allergies
- Animal allergies
What are the 4 types of hypersensitivity reaction?
ABCD
Type 1: IgE antibodies trigger mast cells and basophils to release histamines and other cytokines. This causes an immediate reaction
Type 2: IgG and IgM antibodies react to an allergen and activate the complement system, leading to direct damage to the local cells
Type 3: Immune complexes accumulate and cause damage to local tissues, often autoimmune
Type 4: Cell mediated hypersensitivity reactions caused by T lymphocytes. T-cells are inappropriately activated, causing inflammation and damage to local tissues e.g contact dermatitis
Taking a history of an allergen is one of the most important aspects of diagnosis. What questions do you need to ask?
EATERS
- 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 done to diagnose an allergy if the history is not solid enough?
Skin prick testing and RAST only look at sensitisation, may be sensitised but not allergic
- Skin prick testing: assesses sensitisation
- RAST testing: blood tests for total and specific IgE, assesses sensitisation
- Food challenge testing: gold standard but expensive, done in special unit and gradually increase concentration of allergen
What is the difference between skin prick and skin patch testing?
Skin patch is for allergic contact dermatitis, Skin prick is for food allergies
Skin Prick
Drop of each allergen solution placed at marked points 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
Patch Testing
Could be be for latex, perfumes, cosmetics or plants. A patch containing the allergen is placed on the patient’s skin. The patch can either contain a specific allergen, or a grid of lots of allergens as a screening tool. After 2 – 3 days the skin reaction to the patch is assessed.
What is the difference between skin prick testing and RAST testing?
skin prick testing involves pricking the skin with potential allergens and observing for a reaction, while RAST is a blood test that measures specific IgE antibodies to allergens.
Level of sIgE/size of SPT correlates with likelihood of allergy but do not correspond with severity of allergy
RAST: detects IgE circulating in the blood.
Skin prick tests: detects IgE bound to skin mast cells
What is the issue with RAST testing?
If have an atopic condition like eczema or asthma it is likely to come back positive for everything you test
What are some actual symptoms of a food allergy?
What is the food allergy march?
What is the allergic march?
What effect does temperature have on the allergenicity of foods?
Also remember cross-reactive allergens e.g allergic to walnut may be allergic to brazil or cashew nuts
What are some of the mediators release during an allergic reaction and what do they cause?
What are the different categories of food allergy?
Allergy affects every organ, although skin and mucous membranes are most commonly involved as they are the‘frontier’ between the organism and the environment
How can you tell the difference between IgE and non-IgE mediated food allergies?
Think about timing of onset, duration and organ systems involved
How may Cow’s Milk Protein Allergy present?
Can be IgE or Non-IgE mediated symptoms
- Often causes colic symptoms
- GORD
- Blood/Mucus in stool
- Poor growth
What are some differentials for CMPA?
How common is CMPA and what is the prognosis with this?
- Prevalence of 1-2%
- All present before 12 months old
- Most tolerant by 5 years. Earlier tolerance to baked milk. Small proportion remain allergic
What is Food protein enterocolitis syndrome?
Non-IgE mediated food hypersensitivity
- Symptoms delayed 1-3 hours after contact
- Profuse repetitive vomiting leading to shock
- Occasional watery diarrhoea (affecting 25%)
- Pallor and shock
- Diagnosis frequently missed as appears like sepsis
- WCC can be elevated; CRP normal
How is CMPA managed?
- Breastfed: get mother to remove cow’s milk from diet
- Formula: switch to amino-acid formula or hydrolysed formula
- Every 6 months or so, infants can be tried on the first step of the milk ladder (e.g. malted milk biscuits) and then slowly progress up the ladder until they develop symptoms
How are food allergies managed in general?
- Avoidance of that allergen or food
- Prophylactic antihistamines when contact is inevitable
- Patients at risk of anaphylactic reactions should be given an adrenaline auto-injector
What is the difference between CMPA, cow’s milk protein intolerance and lactose intolerance?
People with cow’s milk protein allergy do not have an allergy to lactose. Lactose is a sugar
Cow’s milk intolerance is not an allergic process and does not involve the immune system. Only GI symptoms
