Allergy and hypersensitivity

Allergy or hypersensitivity occur when the body’s immune system reacts in an exaggerated or inappropriate way to substances in the environment that would not normally cause inflammation and disease.

What causes allergy and hypersensitivity?

Allergy and hypersensitivity are caused by abnormalities in the regulation of the immune system. In some cases the immune system produces a kind of immunoglobulin, or antibody, called immunoglobulin E (IgE). This is the most common kind of allergy.

When IgE is not involved, these kinds of reactions are sometimes referred to as hypersensitivities. Unfortunately, the terms ‘allergy’ and ‘hypersensitivity’ are commonly used interchangeably, which is confusing!

When the mechanism involves the production of IgE, its targets are against substances, known as allergens, which can:

Be breathed in. This can include pollens, house dust mites, moulds or skin/hair/saliva from cats, dogs, rodents or horses.
Be eaten. This includes things such as milk, eggs, fish, shellfish, nuts, cereals or fruit.
Come into contact with the skin, e.g. latex in gloves.
Be injected into the body. This can include drugs such as antibiotics and anaesthetic drugs or insect venom.

Allergic reactions can be relatively mild, such as a hay fever reaction to pollen, or they can be strong and cause a life-threatening reaction called anaphylaxis.

What are the signs of an allergic reaction?

These include:

Sneezing, a runny nose and runny eyes. This is caused by inflammation in the nose, referred to as rhinitis, or when it affects the eyes, it is called conjunctivitis.
Trouble in breathing with a cough, wheezing and breathlessness, such as in asthma. This is caused by inflammation of the airways.
Skin reactions such as eczema or ‘hives’, a raised, red itchy rash also known as urticaria.
Swelling of the deeper layers of the skin, caused by a build-up of fluid. This is known as angioedema.
A sensitivity to specific foods, leading to stomach pains and vomiting.

One particular type of hypersensitivity that can occur in primary immunodeficiency (PID) patients is a reaction to blood products. This can be significant for small numbers of patients treated with immunoglobulin, particularly intravenous immunoglobulin products.