Anaphylaxis Flashcards
What is anaphylaxis?
What can cause it?
An acute (i.e. immediate) type I hypersensitivity reaction that can be systemic
Substances that can pass freely around the body: Penicillin (esp if give IV before Certain drugs Venom in bee/wasp stings Peanuts
Where are mast cells released from and how are they distributed?
What symptoms do they cause in anaphylaxis?
Released as a progenitor from bone marrow
widely distributed in connective tissue (e.g. under the skin) and in association with epithelial mucosae (eg the respiratory and intestinal epithelia)
nausea
Dizziness
hot skin and itching
Breathless and faint
What signalling molecule is involved in anaphylaxis?
What does it act on?
Histamine (inflammatory mediator ) released from mast cell granules
act on blood vessels and smooth muscle which express specific histamine receptors
What class of immunoglobulin is usually involved in anaphylaxis and how does it work?
In normal lymphocyte activation antigen binds to the surface IgM on lymphocytes.
Usually the antibody initially secreted is IgM, and this switches to IgG as the immune response progresses.
In type I hypersensitivity, activation of CD4+ T helper cells (TH2) causes a switch to IgE production
IgE crosslinks on Fc receptors found on mast cells, activating them
Histamine and leukotrienes are secreted causing anaphylaxis
What causes rashes and puffy skin in anaphylaxis ?
skin allergy - stimulation of connective tissue mast cells causes vasodilatation leading to the red colour of the skin rash, and oedema leading to its raised appearance - urticaria
What causes hayfever?
Hay fever and asthma - allergen is inhaled site of action will be mucosal mast cells of the respiratory system
- the nasal passages, the effect will be oedema in the epithelia lining the nose and a consequent general irritation including stimulation of mucus secretion - hay fever (allergic rhinitis)
What causes asthma?
If the allergen reaches the bronchioles of the lungs then there will be contraction of smooth muscle reducing the diameter of the airways, as well as inflammation and increased mucus production.
The result is asthma, with difficulty breathing in and (especially) breathing out
What causes food allergies?
allergen is ingested, then it will act at mucosal mast cells in the intestinal tract.
The stimulation of the associated smooth muscle leads to the vomiting and diarrhoea typical of a food allergy.
If the allergic substance can be absorbed and pass into the blood stream it may also cause skin symptoms (rashes or itching).
Why is adrenaline given to those with anaphylaxis?
Dilatation of peripheral blood vessels causes symptoms of rashes and oedema, but also results in a dramatic drop in blood pressure which can affect organ function: this is known as anaphylactic shock.
An injection of adrenaline will act to constrict peripheral blood vessels and redirect blood to the organs.
What drugs/injection can be given after adrenaline during anaphylaxis?
antihistamines e.g. diphenhydramine and anti-inflammatory corticosteroids e.g. hydrocortisone.
If breathing problems persist, oxygen may be necessary and bronchodilators may be given via an inhaler e.g. salbutamol.
Why do patients with anaphylaxis feel faint?
Due to the dramatic drop in blood pressure (hypotension).
Syncope (fainting) is a temporary loss of consciousness usually related to insufficient blood flow to the brain.
Note that on arrival at A&E, patient can be initially laid down on a couch with his legs raised to counteract this.
Why is blood pressure after anaphylaxis monitored in hospital?
Monitoring of blood pressure provides a reliable indication of recovery.
Occasionally the symptoms return (usually within hours of the initial reaction).
This is known as biphasic anaphylaxis.
Patients with moderate respiratory or cardiovascular compromise should be observed for to up to 8-10 hours before discharge.
What is the long term treatment for anaphylaxis?
Long term treatment is largely by avoiding known allergens.
For those difficult to avoid completely (such as wasp stings, or nuts where small traces may contaminate apparently innocent foods) the patient may be given adrenaline in a self-injection “pen” e.g. EpiPen®, Emerade®.
