The Science Behind Allergic Reactions: What Happens in the Body?

According to Allergy UK, 44% of British adults now suffer from at least one allergy and the number of sufferers is on the rise, growing by around 2 million between 2008 and 2009 alone. The UK has some of the highest prevalence rates of allergic conditions in the world. 

The exact reasons for the rise in allergies are not fully understood; however, there are several factors that are thought to contribute, including changes in lifestyle, diet, environmental factors, and possibly even improved recognition and diagnosis of allergies. 

Understanding allergies

An allergy is your immune system responding to a substance that is usually harmless. Your immune system’s job is to defend your body against things like bacteria and viruses. When you have an allergy, your immune system mistakenly identifies a substance, known as an allergen, as being harmful to you and your body reacts to it. Allergies can vary in severity from mild to severe, and the symptoms can range from being slightly irritating to being life-threatening. 

When a person with allergies comes into contact with an allergen, their immune system releases chemicals, called histamines, which can cause symptoms such as sneezing, a runny or blocked nose, itching, watery eyes, swelling of the lips, mouth, tongue and throat, and in more severe cases, difficulty breathing or anaphylaxis. Anaphylaxis is a severe and potentially life-threatening allergic reaction that can occur rapidly after exposure to an allergen.

Anaphylaxis happens when your body has a serious, life-threatening reaction to something you are allergic to. Anaphylaxis can come on very quickly, so it is important to know what to do in an emergency. It requires immediate medical attention. Anaphylaxis can cause the immune system to release an excess of chemicals which can cause serious symptoms and result in the person going into shock. This is known as anaphylactic shock – it is where there is a sudden drop in blood pressure and there is a narrowing of airways which can block your breathing. If anaphylaxis is left untreated, it can develop into anaphylactic shock and result in serious complications or even death. 

Symptoms of anaphylaxis happen very quickly and include:

• Swelling of the throat and tongue.
• Difficulty breathing or breathing very fast.
• Wheezing, coughing or noisy breathing.
• Difficulty swallowing or tightness in the throat.
• Feeling faint, dizzy or fainting.
• Becoming unconscious.
• Feeling tired or experiencing confusion.
• Skin that feels cold to touch.
• Blue, grey or pale lips or face.
• A drop in blood pressure.

If you suspect someone is experiencing anaphylaxis, you should follow these steps:

• Call an ambulance immediately.
• Administer an adrenaline auto-injector, if available. This is sometimes called an EpiPen. You should use it as directed, which usually involves injecting the person into the outer thigh and holding it in place for 10 seconds.
• If their symptoms have not improved after 5 minutes, use a second adrenaline auto-injector.
• Lie the person down and elevate their legs.
• Stay with the person and monitor their breathing.
• Do not offer any food or drinks as this could potentially make their condition worse.
• Do not ask the person to stand or walk at any time, even if they feel better.
• If the person has been stung by an insect, try to remove the sting if it’s still in the skin.
• Administer CPR if the person has stopped breathing or their heart has stopped.

The immune system’s role

In an allergic reaction, the immune system mistakenly identifies a harmless substance, known as an allergen, as a threat and launches an immune response against it. This response involves the production of antibodies, specifically immunoglobulin E (IgE) antibodies, which are designed to target and neutralise foreign substances like bacteria and viruses. In the case of allergies, IgE antibodies mistakenly recognise allergens as being harmful. 

The first exposure to an allergen sensitises the immune system. During this initial encounter, the immune system produces IgE antibodies specific to that allergen. Upon subsequent exposures to the same allergen, the IgE antibodies recognise it and bind to mast cells and basophils, which are types of immune cells found in tissues throughout the body, particularly in areas prone to allergen exposure such as the nose, lungs, skin and digestive system. 

Following this, there is a:

• Release of chemical mediators – when the allergen binds to the IgE antibodies on the surface of mast cells and basophils, it triggers the release of various chemical mediators, including histamine, leukotrienes and cytokines.
• Inflammatory response – the release of these chemical mediators leads to an inflammatory response, which causes the typical symptoms of an allergic reaction. These symptoms can include itching, swelling, hives, runny nose, sneezing, watery eyes, coughing, wheezing and, in severe cases, anaphylaxis.
• Resolution – once the allergen is no longer present or the inflammatory response is controlled, the symptoms of the allergic reaction typically subside. However, the immune system retains a memory of the allergen, and subsequent exposures can lead to recurring allergic reactions which can sometimes be more severe.

Histamines and inflammation

Histamines are chemicals produced by the immune system in response to allergic reactions. They are released from specialised cells called mast cells and basophils, which are part of the body’s defence system. Histamines play a key role in triggering inflammation, increasing blood flow to affected areas, and promoting other immune responses. Histamines are chemicals that cause a variety of effects in the body, including increasing blood vessel permeability, causing smooth muscle contraction, and stimulating mucus production. These effects contribute to the symptoms experienced during an allergic reaction.

In addition to these immediate symptoms, histamines also play a role in initiating and amplifying inflammation when someone is experiencing an allergic reaction. Inflammation is a complex biological response that helps the body fight off harmful stimuli, such as pathogens or tissue damage. However, in allergies, inflammation occurs in response to harmless substances, which can lead to tissue damage.

Antihistamine medications work by blocking the action of histamines, thereby reducing or preventing allergic symptoms. These medications are commonly used to alleviate symptoms of allergic rhinitis (hay fever), allergic conjunctivitis, hives and other allergic reactions.

Types of allergic reactions

Common allergens include:

• Pollen – these are also known as seasonal allergies. Pollen is a very fine powder which is produced by trees, flowers, grass and weeds used to fertilise other plants. Hay fever, also known as allergic rhinitis, is an allergic reaction to pollen. Many people are allergic to pollen and can experience hay fever symptoms; however, some people have asthma symptoms due to being allergic to pollen. There are approximately 30 different types of pollen that can cause hay fever and allergies. As different plants produce pollen at different times of the year, people experience hay fever and allergy symptoms at different times. However, hay fever and pollen allergies are usually more prevalent in the spring and summer months when the weather is warmer, and plants and flowers release more pollen.
• Dust mites – these are shaped like spiders but are very small. They live on the soft surfaces of your home, for example soft chairs, curtains and carpets.
• Pet dander – this is flakes of skin in an animal’s fur or hair. When someone has a pet allergy, dander is usually what causes the allergy. Pet hair is often grouped with dander as a common allergen.
• Mould – mould and damp are caused by excess moisture. This can be caused by leaking pipes, rising damp in basements or ground floors, or rain seeping in because of a damaged roof or window. Moulds produce allergens, irritants and sometimes toxic substances. Inhaling or touching mould spores may cause an allergic reaction. Moulds can also cause asthma attacks. It is important to address any mould or damp in your home as it can negatively impact on your and your family’s health.
• Certain medications – allergic reactions to different medications can vary widely, and people may react differently to the same medication. It is important to consult with a healthcare professional if you suspect you are experiencing an allergic reaction to a medication.
• Insect bites and stings – there are some insects whose bites or stings are commonly associated with allergies, including bees, wasps, mosquitos, fire ants, bedbugs, ticks and fleas.
• Certain foods – there are 14 common allergens which are listed below.

In the UK, there are 14 allergens listed in food allergen labelling regulations. These are recognised as the most common ingredients that can cause food allergies and intolerances. A food allergy is an immune response to certain proteins. When someone who has a food allergy comes into contact with or ingests an allergenic food, their immune system perceives the proteins as being harmful. 

The 14 most common food allergens are:

• Cereals containing gluten.
• Shellfish.
• Fish.
• Celery.
• Egg.
• Sulphites/sulphur dioxide.
• Milk.
• Mustard.
• Lupin.
• Tree nuts.
• Peanuts.
• Sesame.
• Soya.
• Molluscs.

A food allergy is different to a food intolerance which is where your digestive system finds it difficult to digest that particular food. It means that your gut is sensitive to certain foods and cannot tolerate them. A food intolerance affects your digestive system and causes symptoms which are not life-threatening. Symptoms may begin within a few hours of eating the particular food. A food intolerance does not trigger the immune system like an allergic reaction does and the symptoms are usually less severe. 

Allergic asthma, also known as atopic asthma, is asthma which is triggered by allergens. According to Asthma & Lung UK, 5.4 million people in the UK are currently receiving treatment for asthma, with allergy being the most common cause of asthma, occurring in about 80% of cases. Allergic asthma is a breathing condition whereby your airways tighten when you inhale an allergen. When your allergies combine with the breathing condition asthma, this is called allergic asthma. The allergens are also called triggers as they can set off your asthma and even cause an asthma attack. An asthma attack is when someone experiences severe asthma symptoms. Allergic asthma, also known as allergy-induced asthma, is the most common type of asthma in both adults and children.

Allergies in general can affect what products you use on your skin, what you eat and what you breathe.

Allergic symptoms

The symptoms of an allergic reaction include:

• Skin reactions – this can include itching, hives, a rash, swelling, redness or eczema.
• Respiratory symptoms – including sneezing, runny or stuffy nose, coughing, wheezing, shortness of breath, chest tightness or difficulty breathing.
• Gastrointestinal symptoms – nausea, vomiting, diarrhoea, abdominal pain or cramping.
• Eye symptoms – red, itchy, watery eyes, or swollen eyelids.
• Mouth and throat symptoms – this can include an itchy mouth, lips or throat, swelling of the lips, tongue or throat, or difficulty swallowing or speaking.
• Cardiovascular symptoms – palpitations, rapid heartbeat, low blood pressure or fainting.

The role of genetics

Genetics plays a significant role in allergic reactions, influencing both susceptibility to developing allergies and the specific types of allergens. 

Genetics can play a part in allergies in the following ways:

• Inheritance of allergic predisposition – allergic diseases such as asthma, hay fever, eczema and food allergies tend to run in families. If one or both parents have allergies, their children are more likely to develop allergies as well. This suggests a strong genetic component to allergic predisposition.
• Genetic variations in immune response – genetic variations can affect the functioning of the immune system, influencing how it responds to potential allergens. For example, variations in genes that regulate the production of immunoglobulin E (IgE), a type of antibody involved in allergic responses, can impact someone’s susceptibility to allergies.
• Specific allergen sensitivity – genetic factors can determine which specific allergens someone may be sensitive to. Certain variations in genes associated with the immune system can increase the likelihood of developing allergies to particular substances such as pollen, dust mites, pet dander or specific foods.
• Complex interactions with environmental factors – while genetics play a significant role, allergic reactions are also influenced by environmental factors. Genetic predisposition and environmental exposures, such as pollution, diet and early childhood microbial exposure, contribute to the development and severity of allergic diseases.
• Gene-environment interactions – some genetic factors may interact with specific environmental triggers which may increase the risk of allergic reactions. For example, a genetic predisposition to allergic asthma may interact with exposure to environmental pollutants like cigarette smoke, making symptoms worse.
• Treatment response variability – genetic factors can also influence how people respond to allergy treatments, including medications such as antihistamines, corticosteroids and immunotherapy. Variations in genes involved in drug metabolism, immune response and inflammation can affect treatment efficacy and potential side effects.

Treatment for allergies

Treatment for allergies usually depends on the severity of symptoms and the specific allergen that is triggering the reaction. 

Some common approaches to managing allergies include:

• Avoidance – the most effective way to manage allergies is to avoid exposure to the allergen whenever possible. This might involve staying indoors during high pollen seasons, using allergen-proof covers on pillows and mattresses for dust mite allergies, or avoiding certain foods for food allergies.
• Medications – antihistamines can help to relieve symptoms such as sneezing, itching and runny nose by blocking the action of histamine, a chemical released during an allergic reaction. Decongestants can reduce nasal congestion by narrowing blood vessels, but they should be used for short periods because prolonged use can lead to rebound congestion. Nasal corticosteroids can reduce inflammation in the nasal passages and relieve symptoms like nasal congestion, runny nose and sneezing. Leukotriene modifiers can help block the action of certain immune system chemicals involved in allergic reactions. Allergy injections, also known as immunotherapy treatment, involve gradually exposing the patient to increasing doses of the allergen to desensitise the immune system and reduce the severity of allergic reactions over time. This is often recommended for individuals with severe allergies or those who don’t respond well to other treatments. Emergency epinephrine is used for people with severe allergies, particularly to foods, insect stings or certain medications. This medication can be life-saving in the event of a severe allergic reaction.

Some people find relief from allergies through natural remedies such as nasal rinses, herbal supplements like butterbur or quercetin, and acupuncture. However, it is essential to consult with a healthcare professional before trying any natural remedies, as they may not be suitable for everyone and could interact with other medications.

Allergy UK is the leading national charity for people living with all types of allergies. They work with government, professional bodies, healthcare professionals and corporates to help improve the lives of the millions of people with allergic disease.