Causes of allergies
There are different types of triggers for different types of allergies. For example, to cause hay fever and asthma, an allergen needs to be small. It also must be able to spread through air. This is called an aeroallergen. Other allergens can be ingested or injected.
The common inhaled triggers for allergies are as follows (note that each person has their own unique set of triggers, which is why testing is so important):
Common outdoor respiratory allergens include grass or tree pollens that are spread by the wind. These cause hay fever and/or asthma attacks.
Indoor allergens include dust mites, moulds and animal dander. Dander is the flakes of dried animal skin, saliva and urine that cause an allergic reaction. Sometimes it’s wrongly assumed that people who are allergic to pets are allergic to their fur or hair.
House dust mites
The house dust mite is a minute, eight-legged creature that lives in warm, humid home environments such as bedding, carpeting and fabric furniture. They’re too small to see with the naked eye, being only one third of a millimetre in length.
Dust mites feed on the dead skin scales of humans. Because mites like warm temperatures and humidity, they’re more common in homes near the sea, but they’re also found inland.
Allergy to house dust mites is common, and it’s thought that about 10% of the general population is affected. About 30% of all allergic individuals react to dust mites, making it the single most common allergen in South Africa.
Dust mites can cause asthma, hay fever and eczema, or make these allergic diseases worse/more difficult to control.
Pollen is the very fine powder that comes from trees, grasses, flowers and weeds. Wind and birds carry pollen from tree to tree to fertilise them.
Plants with brightly coloured flowers and sweet smells hardly ever cause allergy symptoms. That’s because insects and birds carry the pollen from these plants (rather than the wind). The pollen from these plants is very large and doesn’t spread in the air as easily as the pollen from plain-looking, wind-pollinated plants.
Different plants are found in different parts of South Africa. The plants in coastal areas differ from those inland, while the plants in urban areas differ from those in rural areas.
Grasses that cause pollen allergy include Bermuda, rye and wild oat grass. Trees that cause pollen allergy include plane, oak and cypress. Weeds that cause pollen allergy include plantain and daisy (cosmos). Each plant has a specific time of year when it produces pollen.
Moulds are fungi that release tiny particles, called spores, into the air. Spores may cause allergic symptoms in people when they’re inhaled. The black spots on the walls and ceilings of damp rooms are moulds, as are the white and black furry layers on decaying bread and other foods. Even mushrooms are a type of fungus.
Mould spores are released throughout the year and are found in both damp indoor and outdoor environments. Moulds are commonly found in kitchens (especially fridges), bathrooms, the soil of house plants, and in areas where humidifiers and tumble driers are used.
Outdoor moulds live in rotting leaves, grass cuttings, compost heaps and rotting seaweed. Moulds love warm, humid places and spore counts tend to peak in spring and autumn. They’re found in higher concentrations in coastal areas.
Tiny microscopic protein particles are present in the scales of skin (dander) that fall off animals. These proteins can set off allergic symptoms when breathed into the nose and lungs, or deposited in the eyes of people who are allergic to them.
This type of allergy is most commonly seen with cats and dogs, but it can also develop with rats, mice, guinea pigs, rabbits, hamsters and birds, as well as farm animals such as horses, cows, chickens, ducks and geese.
There are over 3 500 cockroach species globally, with three common ones found in South Africa – German, American and Oriental cockroaches. In South Africa, people living near the coast are more likely to encounter cockroaches.
Cockroach allergens are found in dried cockroach bodies, scales, hairs and faeces. People with cockroach allergy are often also allergic to other insects such as house dust mites. The highest levels of cockroach allergen are found in kitchens.
Reviewed by Prof Mike Levin, Head of Division of Asthma and Allergy at the University of Cape Town. MBChB; FCPaed; MMed; PhD Diploma Allergology; EAACI UEMS Exam in Allergy, FAAAAI, FACAAI. March 2018
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