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What is an allergy?

An allergy is an adverse reaction to a protein in our environment, such as those found on pets, and in pollen or nuts. These proteins are called allergens and are normally harmless.

In people with an allergy, the body reacts to a specific allergen by releasing histamine from mast cells in the skin, lungs, nose or intestine. This causes inflammation and swelling.

Symptoms can include itchy skin, tissue swelling and wheezing. In severe cases it can lead to full-blown anaphylaxis or even death.

Common allergic diseases include hay fever, asthma, eczema and urticaria.

Some people get allergic conjunctivitis, while others react adversely to medication, insect stings or latex.

Food allergy and intolerance to food additives are relatively uncommon causes of allergic reactions

Allergens to be aware of:

  • Grass and tree pollens
  • Dust mites (living in and feeding on house dust)
  • Food (cow's milk, hen eggs, wheat, soya, seafood, fruit and nuts)
  • Fungal or mould spores (in the bathroom and other damp areas)
  • Medication (penicillin, aspirin, anaesthetics)
  • Nickel, rubber, preservatives and hair dyes (skin contact allergens)
  • Pet skin flakes or dander (cat, dog, horse or hamster)
  • Wasp and bee stings

What causes allergies?

Some families have a predisposition to allergies, known as atopy. This has shown an epidemic rise over the past four decades.

The reasons why are poorly understood. We know some families are genetically programmed to develop allergies, but this can't be the full story. Things that promote allergies must have been added to our environment, while others that previously protected us against allergies must have been removed.

There's growing evidence our fight against infectious diseases and increased personal cleanliness may have interfered with the workings of our immune system.

Global warming has also had an impact, with changing patterns of natural vegetation and more profuse pollen production.

How allergies develop

At birth, the immune system switches to be either allergy prone (TH2) or non-allergy prone (TH1), depending on genetics and environment.

TH stands for T helper type white blood cells. TH1 immunity is good for fighting bacteria and viruses, and protecting against allergies. TH2 immunity is good at fighting parasite infections, but makes us more vulnerable to develop allergies.

If there's a family history of allergies, a child is much more likely to switch on TH2 immunity.

This promotes the manufacture of excessive amounts of allergy-related immunoglobulin E (IgE) in the bloodstream.

This IgE latches on to harmless allergens and triggers allergic reactions.

If an inhaled pollen micro-particle gets attached to IgE in the nasal membranes, for example, this combined IgE/pollen complex causes mast cells to release naturally occurring defence chemicals called histamine.

This leads to profuse nasal itching, tickling, sneezing and a watery mucus discharge.

Who's affected?

Atopy in parents or siblings is a strong indicator of allergy risk. Allergies are likely to occur in atopic families where there's early childhood exposure to certain allergens.

Men are more likely to become allergic and an allergic mother who smokes puts a child at even greater risk.


  • Children from non-allergic families have a 12 per cent risk of developing an allergy
  • If one parent has allergies, this risk increases to 20 per cent
  • If both parents have allergies, the risk is more than 40 per cent
  • If both parents have the same allergy (such as asthma, hay fever or eczema) the child has a 70 per cent risk of having the same allergy

Other factors that may promote allergies include:

  • Birth by caesarian section
  • Frequent courses of antibiotics
  • Coming from a smaller family, with just one or two children
  • Passive cigarette smoke inhalation
  • Being overweight - obese children are more prone to asthma

A baby's environment during the first year is important. Early low-dose exposure to dust mites, pollens, pets and certain foods increases the likelihood of becoming allergic.

On top of that, our relatively affluent lifestyles - centrally heated homes, regular use of antibiotics and processed or exotic foods in our diet - seem to encourage allergy.

Can it be avoided?

A number of factors reduce your risk of developing allergies:

  • Being born into a family with no history of allergies
  • Being breastfed exclusively for the first four months of life, with a mother who avoids egg, nuts and cow's milk while breastfeeding
  • Early exposure to good probiotic bacteria in the infant diet
  • Plenty of vitamins C and E, and omega-3 polyunsaturated oils
  • Having two or more older brothers and sisters
  • Living on a livestock farm and getting grubby playing in the farmyard

Although breastfeeding hasn't been convincingly shown to reduce inhalant allergies or asthma, it transfers protective IgA antibodies to the baby and delays the potential onset of cow's milk allergy by deferring the introduction of cow's milk formula.

Allergic march

The term 'allergic march' is used to describe the progression from one manifestation of allergy to the next over a period of time.

For example, many children under age three have eczema and food allergy. As this improves, they develop asthma. Then, as their asthma begins to settle down, they start to be troubled by allergic rhinitis and hay fever in their teenage years.

Multiple allergies

A small group of highly atopic individuals develop severe allergies from an early age. They may have infantile food allergies (commonly cow's milk, egg and nuts) usually associated with extensive eczema.

Many have cross-reactions to other foods - latex allergy may react with avocado, banana, kiwi and chestnuts, for example. They then develop childhood allergic asthma, allergic rhinitis and remain highly allergic to numerous foods and environmental allergens.

They need ongoing supervision at a combined allergy care clinic under the care of a consultant immunologist, dermatologist, dietician, chest physician, paediatrician and ear, nose and throat specialist.

The vast majority of people with allergies have only a few allergies, which are well controlled by specific allergen avoidance and regular long-term allergy preventer medication.

There's another group of people who apparently react to traces of everyday household and industrial chemicals, but their symptoms aren't typical of allergies.

Often non-medically qualified practitioners will confirm these 'sensitivities' using unproven testing methods.

People can become so incapacitated by fear of a reaction they're no longer able to work or leave their homes. In many instances, there's some past psychological trauma and what they're experiencing isn't an allergy.

This article was last medically reviewed by Dr Adrian Morris in September 2007.
First published in September 1999.

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