Updated 01 June 2017

Causes of allergies

The repeated exposure to an allergen may trigger the immune system to form the antibodies that cause the allergic reaction to the specific allergen.


 The causes of most people’s allergies are not known. However, an allergy usually begins with sensitisation when the person is exposed to an allergen.

This period may last from several days to a few decades. The repeated exposure to an allergen triggers the immune system to form the antibodies that cause the allergic reaction to the specific allergen.

Allergies are classified according to the substances causing them or the parts of the body they affect.

Respiratory allergies

Allergic rhinitis (nasal allergy or allergies of the nose)

Breathing air that contains an allergen causes allergic rhinitis, the most common respiratory allergy. It often begins around the age of two years, but the most common age when this allergy to airborne allergens begins is the late teens or early 20s. Allergic rhinitis is also sometimes called hay fever. Other respiratory allergies include those that affect the sinuses (allergic sinusitis), bronchial tubes (asthma) and lungs (hypersensitivity pneumonitis).

Allergic rhinitis is usually seasonal. The most common allergens are grass or tree pollens spread by the wind. Hay fever is therefore a misleading name, since it is not caused by hay (it also does not produce fever). Perennial allergic rhinitis, which occurs all year round, is usually a reaction to indoor allergens such as moulds, dust and dust mites, as well as animal dander. Dander is the flakes of dried animal skin, saliva and urine that cause an allergic reaction. Sometimes it is wrongly assumed that people who are allergic to pets are allergic to their fur or hair.

Rhinitis of pregnancy

Pregnant women sometimes have some of these symptoms without suffering from allergic rhinitis. This is called rhinitis of pregnancy. The cause is unknown, although hormones released during pregnancy may be the cause of swelling in the mucous lining of the nose. The higher circulating blood volume during pregnancy – 40% above the levels of non-pregnant women – may also congest the nasal passages.


The main causes of asthma, an abnormal reaction in the lower airways, are environmental exposure and allergies to allergens such as pollen, mould spores, animal dander and dust mites. Other allergies, e.g. eczema and allergic rhinitis, are linked to asthma, especially in children.

Skin allergies

Contact dermatitis

This skin allergy is characterised by itchy rashes and is more common in adults. It is caused by contact with a natural substance such as cat fur or a manufactured item such as soap that becomes an allergen.

Atopic dermatitis or eczema

Although it has no known cause, heredity is thought to play a role. Children who have a family history of allergies are prone to eczema, as are those who are allergic to milk, eggs, peanuts, soy, wheat or fish. Children who suffer from eczema are also more likely to develop other allergies later in life, such as allergic rhinitis or asthma. This syndrome is called atopy.

Hives or urticaria

Lasting for minutes or days, hives (also called wheals or welts) are large, itchy red bumps on the skin. Hives can also last for longer (months to years), and are then referred to as chronic urticaria. These hives can be one or many bumps, and vary greatly in size and shape.

Although they more commonly appear on the arms, legs and trunk, they can appear anywhere on the body as a result of the body’s reaction to factors such as certain foods, pollen, animal dander, drugs, insect bites, cold, heat, light or emotional stress. The release of histamine from mast cells in the skin causes the hives to form, but often no cause is found.

Food allergies

More than 70% of people who have a food allergy are younger than 30 years. Adults account for only about one to two percent of people who are allergic to food or some substance in food. Although one in 12 children are suspected to have a food allergy, only one in 25 children are usually confirmed as having a food allergy.

Any food may cause an allergic reaction, but the prime causes are proteins in cow’s milk, egg white, peanuts, wheat and soybeans. Other less common causes include berries, shellfish, maize, beans, gum arabic, and the food colorant tartrazine, which is added to some green, orange and yellow foodstuffs. An allergic reaction to food usually occurs within half an hour of eating it, but can also occur within only five or 10 minutes or after four to six hours.

It is sometimes difficult to establish which allergens are responsible for a food allergy, because reactions to certain foods are sometimes delayed or they may be caused by food additives or eating habits.

Drug allergies

The most common allergy in this category is a reaction to drugs in the penicillin family. Other drugs that can trigger allergic reactions include:

  • Sulphas 
  • Barbiturates 
  • Anticonvulsants 
  • Insulin 
  • Local anaesthetics 
  • Dyes injected into blood for X-ray purposes 
  • Aspirin 

Insect sting allergies

It is thought that people who have allergies such as food, drug or respiratory allergies may possibly be more prone to insect sting allergies. The venom in the stings of bees, wasps, hornets, yellow jackets and fire ants is the common allergen triggering this type of allergic reaction.

If people know they have an allergy to insect stings and that they can easily come into contact with the venom, it is best for them to carry a single-dose injection kit of adrenaline. Injecting themselves with the adrenaline will help counteract and alleviate allergic symptoms until they can receive medical attention. They should consult their doctor to see if immunotherapy is indicated. 

Read more: 

Treating allergies 

Risk factors for allergies 

Diagnosing allergies

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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