Introduction to Allergies

Question 1

What is an allergy?

Answer 1

Allergies are powerful immune responses to an antigen. The antigen is often harmless to the majority but over- reaction by the immune system may cause damage to the body.
Routes of hypersensitisation to an allergen include:
* Skin contact - examples include latex, animal dander and toxic plant material
* Injection – bee stings are a prime example here
* Ingestion – well known allergens include nuts and shellfish but wheat, eggs, soy and dairy can present issues
* Inhalation – pollen, animal dander, dust and mould feature here

Initial exposure to allergens sensitizes the
individual, with subsequent exposure triggering a significant and disproportionate response. This is considered a Type 1 hypersensitivity reaction and is thought in some cases to arise in individuals who produce excessive IgE

Question 2

How is immune response mediated?

Answer 2

• Immune responses have a role to play in the development and perpetuation of the allergic response.
• There are two types of immunity – innate (humoral) immunity, that which we are born with and acquired immunity which we develop over time and exposure to antigens.
• There are also two pathways by which immunity is mediated - cell mediated involving T lymphocytes and antibody mediated involving B lymphocytes.

How cell-mediated immunity works
* Also known as antigen presentation
* Antigen: A foreign body which triggers an immune response
* T-lymphocytes, which are made in bone marrow, have been activated in the thymus and are released into the circulation
* For the T-lymphocytes to get ‘sensitised’ to their antigen, the antigen must be presented to the cell, usually by a dendritic cell or macrophage (the macrophage engulfs the antigen)

How antibody-mediated immunity works
* B-lymphocytes, which are made in
bone marrow and matured in the
lymph nodes, are fixed in lymphoid tissue (spleen and lymph nodes). They can recognise and bind antigens without prior presentation (i.e. no macrophage necessary). Once the B-lymphocyte has bonded to an antigen, a helper T-lymphocyte stimulates it to enlarge, then divide, proliferate and
differentiate.
* Two types of cells are produced: plasma cells
which secrete a single antibody directly into the blood (these are short lived cells) and
memory B cells which remember the antigen in order to secrete more plasma cells for future
encounters

Question 3

What is Atopy?

Answer 3

Atopy is a genetic predisposition to form excessive IgE
* generalised and prolonged hypersensitivity to common environmental allergens (e.g. pollen and house dust mite)
* one or more of a group of diseases tend to manifest including urticaria, food and other allergies, asthma, hay-fever and a very distinctive presentation of eczema
* atopic conditions tend to run true to type within each family

Question 4

What is Anaphylaxis/ angioedema?

Answer 4

Acute, potentially fatal, multiorgan system reaction

Caused by the release of chemical mediators from mast cells and basophils.
Physiologically the body’s responses are
- smooth muscle spasm in the respiratory and gastrointestinal (GI) tracts
- Vasodilation and increased vascular permeability leading to - hypotension, cardiac arrhythmias, syncope, shock
- Increased mucous secretion, increased bronchial smooth muscle tone, airway oedema

Signs and symptoms:
Urticaria/angioedema, laryngeal oedema, flushing, pruritus, bronchospasm, abdominal cramping with nausea, vomiting and diarrhoea