Updated 01 June 2017

What are allergies?

Allergic persons form antibodies against substances that are harmless to most individuals, such as certain types of food and pollen. These substances are called antigens or allergens.


The function of the immune system is to protect the body against foreign substances or hostile invaders such as viruses, bacteria and harmful chemicals. When such a substance enters the body, the immune system reacts by producing proteins called antibodies or by sending specific white blood cells called eosinophils to that area.


Eosinophils attack infection, viruses, bacteria and other foreign substances. It is also these white blood cells that cause the immune system to overreact while they "attack" an invader, which usually is a substance harmless to other people.

The immune system produces five kinds of antibodies, each with its own function. Allergies involve the IgE antibody. Allergic persons form IgE antibodies against substances that are harmless in most individuals, such as certain types of food and pollen, called antigens or allergens.

Antibodies against harmful substances are produced by B lymphocytes, the body’s white blood cells that are responsible for fighting infections. Antibodies attach themselves to the substances and set off a reaction that destroys them. Antibodies usually leave harmless substances alone.

Once the IgE antibodies are formed against a specific allergen, they bind to special mast cells which are found in the skin, lungs, nose and intestines and are also known as basophils which circulate in the blood. This process is called allergic sensitisation.

Anaphylactic shock

When a person is exposed to that specific allergen again, the IgE antigens attached to mast cells and basophils bind to the allergen, causing a reaction that causes the mast cell to burst. When the mast cell bursts (also called degranulation of mast cells), it releases chemicals such as histamine. 

These chemicals make the blood vessels dilate and smooth muscles contract. For example, the smooth muscles of the bronchi contract and the air passage narrows, which may lead to asthma. These chemicals also attract eosinophils to that area. The eosinophils release chemicals that neutralise the chemicals released by the mast cells.

When the allergic reaction is severe, e.g. to a food allergen such as shellfish or a medicine such as penicillin or aspirin, so much histamine is released that the person’s blood pressure drops, blood circulation slows down and the bronchi contract, causing respiratory distress. Such a severe allergic reaction is called anaphylactic shock and may cause death. Fortunately this is rare. 

Read more: 

Causes of allergies 

Diagnosing allergies  

Preventing allergic reactions

Revised and reviewed by Professor Sharon Kling, Allergologist, Clinical Head of the Paediatric Allergy Clinic at Tygerberg Hospital and Associate Professor of Paediatrics, University of Stellenbosch. March 2015.


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Dr Morris is the Principal Allergist at the Cape Town and Johannesburg Allergy Clinics with postgraduate diplomas in Allergology, Dermatology, Paediatrics and Family Medicine dealing with both adult and childhood allergies. obesity and diabetes societies and runs a trial centre for new drugs.

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