The basic cause of classical allergy is an immune reaction involving mast cells and IgE antibodies.
Mast cells are plentiful in the lining of the nose, the airways, and the digestive tract. They have counterparts in the blood, called basophils.
Seen under the microscope, both mast cells and basophils look very granular inside.
The granules are tiny storage compartments, containing stockpiles of messenger chemicals, notably histamine.
Histamine causes several different reactions:
Contraction of muscle around the airways. This reduces the diameter of the airway,producing an asthma attack
Increased leakiness of the smallest blood vessels, allowing fluid and immune cells to escape into the surrounding area – for example, the skin or airway lining
If sufficient histamine is released into the blood, a drastic fall in blood pressure, due to widespread opening of blood vessels, and leakage of fluid into the tissues; this occurs in anaphylaxis.
As a result of these two above effects, local swelling (called oedema or angioedema) and irritation – in the skin this is experienced as urticaria, or nettle rash, in the nose it causes blockage, itching and sneezing
As a result of these two above effects, local swelling (called oedema or angioedema) and irritation – in the skin this is experienced as urticaria, or nettle rash, in the nose it causes blockage, itching and sneezing
Widening of blood vessels
Histamine is released when mast cells are activated, a process called degranulation because the cells discharge their storage granules.
Mast cells release other substances at the same time, some of which attract more immune cells to the area, causing more inflammation. They help to produce a ‘Late Phase Reaction’ which occurs after the initial allergic reaction has died down, and lasts about 24 hours. Once activated, mast cells also start making messenger chemicals called leukotrienes which are highly inflammatory.
What causes a mast cell to degranulate?
The answer is found on the surface of the cells, where the allergy antibody, IgE, sits. One end of the IgE molecule is bound to the mast cell, and the other end can bind to the allergen concerned. In someone allergic to egg, for example, egg allergen will bind, with great specificity, to egg-specific IgE antibody.
For the receptors to pass a message to the mast cell there have to be two IgE antibodies specific for the same allergen on the mast cell – and the allergen has to bind to both these IgE molecules, cross-linking them. This is the ‘go’ signal for the mast cell to degranulate.
Recognising anaphylaxis
In the early stages of the reaction it may be difficult, even for a doctor, to be sure it really is anaphylaxis. Fainting or a panic attack can look very similar. But those who are simply fainting are pale and have a slow pulse, whereas those suffering anaphylaxis are usually pink in the face and have a fast pulse. If someone is just fainting, their blood pressure normalises and they start to recover when they lie down, but this is not true for anaphylaxis