Anatomy of an allergy

By Smriti Daniel

Allergens are everywhere – the dustmite faeces in your cotton pillow or the clump of pollen riding on the breeze will do it for some. For others, the shrimp in their omelette or the sweet flesh of a woodapple are enough to trigger severe allergic reactions.

The symptoms come in a variety equal to the allergens themselves – some of us will sneeze till we’re dizzy, others will burst into hives. We’ll scratch and vomit and struggle to breathe. A rare few might even find themselves near death. This week, MediScene speaks to Dr. Neelika Malavige, Senior Lecturer and specialist in Immunology, University of Sri Jayewardenapura, about allergies, their most common forms and how to spot them.

What is an allergy, what causes it?

Our immune system is there to protect us against infections and when germs enter our body it reacts to it and eliminates it. But in some people, their immune system starts to react against harmless environmental particles, which cause allergies. When it reacts against food particles we get food allergies, when it reacts against particles we inhale, we get asthma and allergic rhinitis (hay fever). Some food and inhalant allergens can cause eczema, which is an itchy rash that develops mainly behind knees and elbows. Some people are allergic to insect stings (fire ants, wasps, bees etc…)

Are allergies on the rise in Sri Lanka?

Our Annual Health Bulletin shows that there has been a steady rise of patients with asthma. The prevalence of other allergic diseases has not been recorded over time, so it is difficult to comment. However, data from other countries have shown that there has been a dramatic rise in allergic diseases in Western countries, India, South East Asia and even Africa.

What are the most common allergies?

In Sri Lanka we have found that house dust mite is the commonest allergen in individuals with asthma and allergic rhinitis. Around 80% of such individuals are allergic to the faeces of this insect, which is found in household dust. However, allergies to other inhalant allergens such as cockroach, molds (fungi) and storage mites are also common.

The food allergy patterns in Sri Lanka are different to those in the West. The number one food allergen in children appears to be cow’s milk, followed by egg. Adults and children were found to be allergic to a wide variety of food such as pork, beef and fruits such as woodapple, which are uncommon in the West.

What are the symptoms of an allergy attack?

The symptoms of an allergy attack vary according to the system involved. In patients with asthma, when they are exposed to the allergen they get wheezing. Individuals with allergic rhinitis get sneezing, dripping of the nose, nose block etc… Those with food allergies get symptoms immediately after consumption of the food and may get skin involvement such as itchy raised bumps on the skin (urticaria/hives); respiratory symptoms such as wheezing, sneezing, difficulty in breathing, itching of the tongue; gastrointestinal symptoms such as vomiting, abdominal pain and diarrhoea and cardiovascular symptoms such as dizziness, lowering of blood pressure and finally loss of consciousness.
They may have some symptoms or all symptoms. Some reactions to food may be life threatening unless immediate medical treatment is not given. Individuals with insect venom allergy may have similar symptoms.

However, rashes (urticaria/hives) that occur many hours after consumption of a food are not due to a food allergy. Symptoms of food allergy almost always occur at least within 2 hours of consumption of food. A lot of people with itchy rashes (urticaria/hives) that occur everywhere in the body on most days of the week have a condition called chronic urticaria. This is usually due to reaction of the immune system to self protein or self molecule (autoimmune reaction), rather than an allergic reaction (immune reaction to something in the external environment). Most patients with this condition, because of the misconception that it is a food allergy, sometimes suffer for years excluding various foods from their diet without any relief.

Is it always easy to spot an allergy or are many people unaware they have one?

Most people would realise that they are allergic to something if it’s a food allergy, because symptoms appear almost immediately. Those who are allergic to inhalant particles would sometimes not know because it would be difficult for one to know whether they are allergic to cockroach for instance.

Can allergies be life threatening?

Yes they can be. Life threatening allergic reactions are called anaphylaxis. Allergies to food, drugs and insect venom have resulted in many episodes of anaphylaxis and death. Anaphylaxis is a medical emergency. A patient can die in few minutes unless immediate treatment is not given. However, these episodes can be prevented with proper management and precautions.

How are drug allergies identified?

Allergies to drugs are due to complex mechanisms. Some drug reactions are very straightforward as people get symptoms as soon as they take the drug. Some allergic reactions to drugs may take several days to appear and then it might be difficult to decide which drug it is if the patient is on multiple drugs. Allergy testing for drugs can only be done for certain types of drugs.

Why are some people more vulnerable to allergies? Is there a genetic component?

Allergies do run in families. Individuals with asthma, allergic rhinitis, atopic dermatitis or food allergy are known as atopic individuals. If one parent is atopic the chances of their offspring developing an atopic disease is approximately 25%. Genes do play a major role, but so does the environment.

Does exposure to dander etc during infancy make children more likely to develop an allergy?

This is a controversial area. Some studies have shown that exposure to high levels of house dust mite during infancy may predispose to allergy to this. However, some studies have shown that exposure to high levels of cat dander actually reduces risk of developing allergy to cat dander. So this is a controversial area at the moment.

What are some of the misconceptions about allergies you’ve come across in your practice?

One of the main misconceptions is that itchy bumps that appear on the skin (urticaria) that persists for weeks and sometimes years, is due to food allergy. As a result some patients take extreme measures of avoiding all food protein and still have the symptoms. However, this condition, which is known as chronic urticaria is rarely due to food allergy, but is due to an immune reaction to self proteins (autoimmune reaction).

Another misconception that most of the general public seem to have is that they do not realise that some allergies can in fact be life threatening. If someone does have a ‘true’ food allergy, it is best to avoid it. Consumption of the food that the patient is allergic to can result in severe allergic reactions. Fortunately, most children grow out of allergies as they grow older. So if a nine- month old infant is allergic to cow’s milk, it is very likely to disappear when the child is 2 years old. Unfortunately, allergies in adults and certain types of allergies (allergies to nuts) tend to persist.

How does the skin prick test work? How do you measure the strength of a person's response?
Skin prick tests are used to find out the possible allergens in patients with asthma and allergic rhinitis. In patients with food allergy, they are helpful to confirm the food allergy.

They may also be beneficial in patients with atopic dermatitis to find out if allergy to cow’s milk or egg could be aggravating factor.

In these tests, a drop of the allergen solutions is placed on the patient’s skin and a small prick is made. If a person is allergic to something, they will get a bump for the specific tested allergens. To find out if a test is positive or not, a negative control (saline) and a positive control (histamine) is always used. Depending on the reactions to these, the doctor decides if the patients bump to an allergen is significant or not. These tests are painless and take about 20-25 minutes to find out the allergy.

However, testing individuals in the absence of symptoms of allergic diseases is not recommended because false positives can occur. These tests are also unsuitable for patients with chronic urticaria (described above), as they could test positive even in the absence of allergy due to the hype reactive nature of their skin.

What are the other ways of diagnosing an allergy?

Allergies can also be diagnosed by a blood test which measures antibodies to specific allergens. The blood test to measure an allergen in the blood is called an immunocap method, which is currently unavailable in this country. Other blood tests to diagnose allergies are less specific than skin prick tests. The most important this is that all allergy tests should only be done if the patients have symptoms of an allergic disease because of false positive reactions in patients who are not really allergic to anything.

Can allergies be cured completely? Do some people become more or less allergic over time?

Some people do overcome their allergies with time. Especially children tend to outgrow their allergies. However, allergies in adults are less likely to disappear. Those with severe food or insect venom allergy should be prescribed emergency medicine such as adrenaline. These life saving injections are usually prescribed to all those who are likely to develop a life threatening allergic reaction and they will be soon available in Sri Lanka. These are very simple to use pen devices and can be administered by any lay person in an emergency, in order to save the life of the allergic person.

Some allergies can be cured completely such as those to insect venom by immunotherapy. Immunotherapy is also available for some inhalant allergies in other countries. However, for allergen driven diseases such as asthma, allergic rhinitis and atopic dermatitis, the treatment is rather complex. These diseases are best managed by physicians/paediatricians/dermatologists who have a lot of experience in treating such patients.

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