There are many reasons why we, as humans, could experience a reaction to something in our environment – for example a food, pollen, drug, insect or mould.
One obvious reason is that the substance could be poisonous. If something is poisonous, we experience what is called a “toxic reaction”. With toxic reactions, everyone who is exposed to the substance will have a reaction.
Poisons don’t discriminate – they may be more or less severe in certain individuals, but everybody will respond in some way.
On the other hand, some people will have a reaction to a substance at an amount other people can usually tolerate. This is called a “hypersensitivity”. When you have a reaction to substances in the environment that are harmless to most other people, you’re said to have a “hypersensitivity reaction”.
This type of reaction could happen for different reasons in different people – and this is where the two terms, “allergy” and “intolerance”, come into play.
Allergy and intolerance are both hypersensitivity reactions, but they differ greatly. In an allergy, the immune system causes the reaction; in an intolerance, the reaction is caused by something else – for example, an enzyme that’s missing in your body or not active enough. This might seem like a small distinction, but because of this, the reactions are very different.
The allergic reaction
The immune system, which protects the body against viruses, bacteria and other hostile invaders, sits at the centre of an allergic reaction. When an invader enters the body, the immune system reacts by producing proteins called antibodies or by sending white blood cells to that area to kill and remove the invader.
B lymphocytes, the body’s white blood cells, are responsible for fighting infections. They produce five kinds of antibodies, each with its own function. Antibodies attach themselves to the substances and set off a reaction that destroys them. Good antibodies, such as IgG and IgM, usually leave harmless substances alone.
Most of the time, the immune system does a great job of killing and removing invaders, but occasionally it acts against harmless proteins found in the environment. This is what causes an allergic reaction.
An allergen (or antigen) is a substance that’s harmless to most people, but which can cause a reaction in allergic people. Common allergens are pollen, dust mites, pet skin proteins, and certain types of foods and medications.
The first stage of an allergic reaction is when an allergic person comes into contact with an allergen and forms a specific type of antibody called IgE, which can recognise that specific allergen – a process known as allergic sensitisation. This can be picked up in your blood with the help of a specific IgE test, or by performing skin prick tests.
The test therefore picks up sensitisation, and not allergy directly. Sensitisation can lead to allergy, but in some cases the sensitisation is suppressed by other mechanisms in the body. This is called “innocent sensitisation”. If this is the case, the test will show that you’re sensitised, but that you will not actually be allergic.
Once IgE antibodies are formed against a specific allergen, they bind to special cells that are found in your skin, lungs, nose and intestines (called mast cells) or in the blood (called basophils). The antibodies sit on the cells, just waiting for something to happen.
If you’re exposed to the specific allergen again, and you don’t have other mechanisms to suppress a reaction, you’re allergic and not merely sensitised. The next stage of the reaction then takes place: the IgE antibodies that are attached to the mast cells and basophils bind to the allergen, causing the mast cells to burst. When these cells burst (a process called “degranulation” of mast cells), it immediately releases chemicals such as histamine. Later, it causes chronic allergic inflammation.
The immediate reaction makes the blood vessels dilate and the smooth muscles contract. For example, the smooth muscles of the airways contract and the passage narrows, which may lead to an asthma attack.
These chemicals also attract eosinophils to that area. Eosinophils are white blood cells that attack viruses, bacteria and other foreign substances. However it’s also these cells that cause the immune system of an allergic person to overreact while they “attack” an allergen. This can lead to allergic inflammation.
Once inflammation is present, it tends to last for a very long time, unless treated. It makes your whole immune system “twitchy” and likely to over-react again, and again, setting off a vicious cycle.
If an immediate allergic reaction is severe, e.g. to a food allergen such as shellfish or a medicine such as penicillin or aspirin, so many chemicals are released from the allergy cells that you not only suffer from a rash, swelling and other less serious symptoms, but also from a tight chest, closing of the throat, difficulty breathing and/or a drop in blood pressure.
This severe allergic reaction is called anaphylaxis, and may cause death. Fortunately, this is rare.
Course and prognosis of allergies
Most people know that allergic disorders tend to run in families, but fewer people know that, as an individual with one allergic disease grows older, they may go through a characteristic progression from one form of allergy to another. This is known as the atopic march.
Allergists now know that people often start with atopic dermatitis and food allergy, and then progress to asthma and rhinitis. This is partly due to the fact that there are common mechanisms behind these different diseases.
However, as more research is done, it’s becoming clear that there are many different types of each of these illnesses, with some of them being more closely related than others.
In general, some allergic diseases settle down, whereas others tend to linger longer or affect individuals for the rest of their lives. This may even differ according to the allergen involved.
For example, people with milk and egg allergy may outgrow this problem early, but people with peanut or fish allergy tend to have it lifelong.
Asthma is an interesting disorder in that some children who wheeze a lot when they’re young never go on to develop asthma in later life. In fact, it’s debatable whether they truly had asthma at all.
In other children with definite asthma, symptoms may go away as the child’s lungs physically grow bigger, only for the asthma to return when they hit their 40s (as their lung function starts to decline).
The search for ways of “switching off” allergies is ongoing, and researches have made significant progress. Immunotherapy is the process by which allergists trick the body into becoming non-allergic to something you’re allergic to.
This can be done very successfully, and researchers believe it’s possible to rid individuals permanently from allergies to bee and wasp venom, and certain inhaled allergens like grass pollen and dust mites.
Immunotherapy can also be used very successfully, on a temporary basis, to treat people who are allergic to various medications. The therapy can make it possible for you to take a course of the medication despite being allergic to it. As soon as you stop taking the medication, your allergy will return.
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 Allergy exam, FAAAAI, FACAAI. March 2018.