What happens to the body during an allergic reaction?

When a person comes in contact with an allergen, the immune system reacts, causing an allergic reaction. But, what exactly happens during an allergic reaction?

A woman having a runny nose due to an allergic reaction

The immune system produces five types of antibodies, namely; IgG, IgM, IgA, IgD and IgE. These antibodies (also called immunoglobulin) are involved in the immune response. They are protective proteins produced by the immune system in response to a foreign substance (called an antigen).

The antibody involved in allergic reactions is IgE. When you are allergic to a substance, your body forms the IgE antibody against that substance. When that substance comes in contact with your body, the IgE launches an attack against it, causing a nasty reaction called an allergic reaction.

Recently, allergy cases have been on the rise in the industrialised world. According to the American Academy of Allergy, Asthma and Immunology, in 2010, 11.1 million visits to physician offices resulted in a primary diagnosis of allergic rhinitis.  

Experts have suggested that one of the leading causes of the increasing allergy rates is hygiene. According to this hypothesis, the living condition in some parts of the world is too clean. The result is that children don’t get exposed to germs that will train their immune system to differentiate between harmful and harmless substances. 

Another suggested cause of the increase in allergies is antibiotics and acetaminophen. Studies have shown that the increased use of antibiotics has led to the rise in health conditions like allergy and asthma. 

What is an allergy?

Allergies are the body’s natural reaction to anything the body views as “a foreign invader”. An allergy is a condition that occurs when the immune system reacts abnormally to a foreign agent. 

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The role of the body’s immune system is to fight against foreign agents, particularly infectious agents like bacteria and viruses. However, in the case of allergies, the body reacts to normally harmless substances such as pollen from plants. 

If a person allergic to pollen comes in contact with it, the person’s immune system will react to the pollen.

What is an allergic reaction?

A substance that causes an allergic reaction is called an allergen. An allergic reaction refers to the way the body reacts to an allergen.

Common allergens include dust mites, pollen, animal dander, moulds, insect venoms (sting), latex and certain foods. 

Allergic reactions cause symptoms in various parts of the body, such as the sinuses, the lungs, the nose, the ears, the throat, the skin, and the stomach lining. 

What happens during an allergic reaction

A series of events occur that result in an allergic reaction. When people are prone to an allergen, e.g. pollen, their immune system identifies pollen as an invader. When such people are exposed to the pollen for the first time, their body responds by producing IgE, which are allergic antibodies. 

The role of the IgE antibody is to find the allergens and get rid of them from the body. This results in the release of a chemical called histamine. Histamine is stored in masts cells (a type of immune system cell) located in the guts, lungs, skin, nose, mouth and blood. 

Histamine plays a central role in the development of several allergic diseases such as allergic rhinitis, atopic dermatitis and allergic asthma. It causes the allergy symptoms experienced during an allergic reaction.

Once released from its granules, histamine causes different effects on the body, such as the dilation of blood vessels, contraction of smooth muscles of the uterus, lungs and stomach, increase in heart rate and stimulation of gastric acid secretion in the stomach.

The following happens to your body when you have an allergic reaction:


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One of the things that happen during an allergic reaction is the appearance of hives. Hives appear as raised spots that are pink, red or skin-coloured. They are itchy and vary in size and shape. 

Hives appear when the body releases histamine as a defence mechanism. The released histamine causes the surrounding blood cells to dilate and increase in permeability. Blood fluid, alongside white blood cells and plasma proteins, leak through the blood vessels causing hives to appear. 

Hives typically don’t last for long; they usually disappear within 24 hours. 

2.Digestive problems

One of the things that happen during an allergic reaction is digestive issues. People who are allergic to food substances often experience digestive problems during an allergic reaction. This is because the antibodies their body developed against that food substance (the allergen) struggle to invade the substance. 

Common digestive problems experienced during an allergic reaction include nausea, abdominal pain, diarrhoea and vomiting.

Most of these allergy symptoms occur within two hours of ingesting the food substance. The allergic reaction may start after about 4 to 6 hours in rare cases. 

3.Runny or congested nose

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The nose does a great job of filtering air that passes through it every second. However, when the nostrils get hit by an allergy, the immune system overreacts, causing a series of allergic reactions. 

The mast cells in the nasal cavity release the chemicals leukotrienes and histamines to fight an allergen. The release of these chemicals can cause the swelling of blood vessels in the nasal cavity and an increase in mucus production, which causes nasal congestion and a runny nose.

4.Anaphylactic shock

An anaphylactic shock can occur in severe cases of allergy. Anaphylaxis occurs when the immune system releases a series of chemicals that can cause the body to go into shock.

Blood pressure declines abruptly during an anaphylactic shock, and the airways narrow, disrupting normal breathing. Anaphylaxis is dangerous. It can cause serious complications which are potentially life-threatening if not treated immediately.

Anaphylactic shock can cause shortness of breath, swelling of the tongue and throat, difficulty in breathing, gastrointestinal problems, a drastic drop in blood pressure and red, itchy skin. The most common anaphylactic reactions are reactions to food, latex, medications and insect stings.

Other symptoms a person may experience during an allergic reaction include watery eyes, tissue swelling, constriction of bronchi.

What to do when a person has an allergic reaction

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If you or someone close to you has an allergic reaction, seek emergency care immediately. 

If it is not a first-time incidence, the chances are high that the person knows what is causing the reaction. Check if the person is wearing an allergy bracelet or ask them what they are allergic to. 

If the allergen is a substance that can be gotten rid of right away, e.g. pollen, go ahead and get rid of it immediately. If the person is not breathing, quickly call for help, give them CPR while the medical team arrives. 

If the person is conscious, place them in a sitting position and encourage them to take deep, slow breaths. If there is an epinephrine auto-injector (EpiPen), use it at the onset of the symptom. An anti-histamine medication can also help relieve the symptoms. 

Note that this is only a first aid tip. The person still needs to get medical attention from a health professional.