Written by Ada’s Medical Knowledge Team
What is a food allergy?
A food allergy develops if the body’s immune system produces an abnormal response to one or more proteins contained in food. 1 This will often lead to unpleasant symptoms, commonly including itching or swelling of the mouth, a raised rash and vomiting.
Food allergies are very common, with 15 million people in the USA alone estimated to be affected by a food allergy of some kind. 2 Any type of food can cause an allergy. However, 90 percent of all allergic reactions to food are caused by 8 foods or food groups: 3
- Shellfish, such as crab, lobster and prawns
- Tree nuts, such as brazil nuts, almonds and walnuts
There are several different types of food allergy, each defined by which part of the immune system is causing the response. 4 While most allergic reactions to food are mild, in some cases, a severe response, known as anaphylaxis, can occur. In cases such as these, urgent medical attention is required.
People who suspect that they or a loved one may have a food allergy should consult a doctor. Diagnosis will usually be based upon the symptoms being displayed, as well as tests, such as a skin-prick or blood test. Additionally, the free Ada app can be used to to carry out a symptom assessment.
Food allergy symptoms
Symptoms of a food allergy can appear within a few minutes to a number of hours after eating food containing the allergen. They can range from mild to severe, and, if anaphylaxis occurs, symptoms may even be life threatening.
Symptoms are also unpredictable and subject to change. What appears to be a mild reaction could become severe very quickly, or a food that causes a mild reaction in one instance could cause a severe reaction when eaten again. 5
Mild to moderate food allergy symptoms
- A tingling and/or itching sensation in the mouth
- Swelling of the lips, tongue and/or throat
- Swelling of the face, particularly around the eyes
- A rash or hives, which may be raised, red, itchy and blotchy
- Nausea and/or vomiting
- Wheezing and/or breathlessness
- Stomach pain
- Nasal congestion and/or a runny nose
- Sneezing and/or dry coughing
- An unfamiliar, strange or metallic taste in the mouth
People experiencing an allergic reaction to food may develop one or more of these symptoms.
If possible symptoms of a food allergy occur, a doctor should be contacted as soon as possible. Additionally, the free Ada app can be used to carry out a symptom assessment.
Severe food allergy symptoms: anaphylaxis
The following symptoms may be a sign of a severe food allergy, otherwise known as anaphylaxis. These symptoms may be combined with symptoms of a milder allergic reaction and in all instances require immediate treatment.
- Difficulty swallowing and/or speaking
- Turning blue and/or generalised flushing
- A drop in blood pressure, which may cause faintness, confusion and/or weakness
- Chest pain
- Heart rate alterations
- Abdominal pain, nausea and/or vomiting
- A sense of impending doom
- Loss of consciousness
There are two main ways in which these symptoms may occur: 6
- Monophasic. Symptoms appear quickly and worsen rapidly. Once treatment has been received, however, symptoms will disappear and not return.
- Biphasic. Mild to severe symptoms will appear, before disappearing for a period of time. This will usually be a couple of hours, but can be as long as 72 hours. After this period, symptoms will return and typically include breathing and blood-pressure symptoms.
Whether the anaphylaxis is monophasic or biphasic, urgent medical attention is required. In most cases, a stay in hospital will be required for observation.
Non-IgE mediated allergic reaction symptoms
Allergic reactions to food are either caused by the antibody immunoglobulin E (IgE) or are non-IgE-mediated. This is covered in further detail in the causes section below.
IgE reactions are the most common, causing the symptoms described above. Rarer, non-IgE-mediated reactions can cause different symptoms, including: 1
- Atopic dermatitis
- Acid reflux
- Loose and/or frequent stools
- Mucus or blood in the stool
- Redness of the anus
- Poor growth in children
People experiencing a non-IgE-mediated allergic reaction may present one or more of these symptoms, which may range from mild to severe.
Food allergy causes
The immune system helps to protect the body against infection and illness. If it detects a perceived threat, it releases proteins, known as antibodies, to neutralize it.
Most food allergies are caused by the immune system mistakenly releasing an antibody called immunoglobulin E (IgE) to combat harmless proteins in food. This is a type-I-allergic reaction. IgE triggers the production of several chemicals, including histamine, the chemical responsible for most of the physical symptoms experienced during an allergic reaction. 7
The production of histamine is usually limited to certain locations, such as the areas around the mouth or throat. When the immune system produces large amounts of histamine and releases it into the blood, this causes anaphylaxis.
Non-IgE food allergies
IgE causes the majority of allergic reactions to food. In some cases, however, a reaction can be non-IgE-mediated, something known as a type-III-allergic reaction. These types of reactions are caused by the immune system releasing different types of antibodies that activate cells, such as granulocytes, a type of white blood cell.
Non-IgE-mediated reactions tend to be less severe and less immediate. They are also typically limited to the skin and digestive system, causing symptoms such as heartburn and indigestion in adults, and diarrhea and acid reflux in babies.
Which types of food cause allergies?
Any type of food can provoke an allergic reaction. In the United States, however, 90 percent of all allergic reactions to food are caused by 8 foods or food groups. These are: 3
- Shellfish, such as crab, lobster and prawns
- Tree nuts, such as brazil nuts, almonds and walnuts
Other foods associated with allergic reactions include: 7
- Sesame seeds
- Celery, celery seeds, celery salt and celeriac
- Fruits and vegetables
- Pine nuts
Good to know: In the case of fruits and vegetables, a person may be allergic to just one item – bananas, for example – or a range. This is also true of meat; a person may be allergic to a single type of meat, such as beef, or several different types. It is also common for people with a fruit and vegetable allergy to be allergic to pollen, too. This is known as oral allergy syndrome.
Precisely why some people develop food allergies and others do not is not known. Certain factors that increase a person’s likelihood of developing a food allergy, however, have been identified.
A person has a higher likelihood of developing a food allergy if a close family member, such as a parent or sibling, has an allergic condition. This allergic condition does not necessarily have to be a food allergy, it could also be a condition like asthma or eczema. Despite this link having been made, precisely which genes cause allergic reactions remains unknown.
Other allergic conditions
Children who have allergic conditions, such as eczema or asthma, are more likely to develop a food allergy later in life. This is also true when reversed: children with a food allergy are more likely to develop another allergic condition, such as eczema or asthma.
The precise reason for this is not known. It is not the case, however, that one condition causes the other. It is more likely that the conditions have a number of shared risk factors.
Food allergy diagnosis
People who suspect that they may have a food allergy should consult a doctor as soon as possible. The initial step will involve the doctor asking a number of questions about the pattern of symptoms being experienced. Questions may include: 1 9
- What symptoms have you experienced?
- What food do you suspect caused the allergic reaction?
- How long after eating the food did symptoms occur?
- How much of the food was consumed?
- How was the food prepared? Could, for example, another ingredient be causing the reaction?
Further questions about a person’s medical history may include:
- Is there a family history of allergic conditions?
- Do you have any other allergic conditions? Eczema or asthma, for example.
Depending on the answers to these questions and a physical examination, the doctor may order a number of tests. Alternatively, the affected person will be referred to a specialist, usually an allergist or gastroenterologist, for further investigation.
This test involves pricking or scratching the skin with a small probe coated with extracts of certain foods. This will usually be carried out on the forearm and is not usually painful.
The foods used will generally be whatever is suspected of causing the reaction, as well as other common food allergens and common non-food allergens, such as pollen. Redness, itchiness and/or swelling will signal a positive reaction. Due to the small size of the area tested, these symptoms will not usually cause any significant discomfort.
Blood tests can be used to measure the number of allergic antibodies in the blood. Blood tests do not need to be carried out by an allergy specialist, but a specialist may be required to interpret the results. This type of test may be used as an alternative to or in conjunction with a skin-prick test.
Other methods used to diagnose a food allergy may include:
Food elimination diet
A food elimination diet involves withdrawing the food thought to be causing the allergic reaction for a period of time, usually two to six weeks, before reintroducing it. If the symptoms disappear during the period of withdrawal and return when the food is reintroduced, this will give a strong indication of the source of the allergy.
A food elimination diet should only be undertaken under the supervision of a doctor or allergy specialist. Completely eliminating a whole food group from one’s diet has the potential for adverse effects, including malnutrition in children. Professionals will be able to advise on how best to handle the dietary changes.
During the diet, food labels should be carefully read prior to eating. The food being avoided may not be an obvious ingredient of a certain dish, so it is important to be diligent. If eating at a restaurant, the affected person should ask a member of staff about ingredients before ordering.
A healthcare provider may request that a food diary be kept. This should include all food and drinks consumed over a period of time, including things such as condiments and candies. This will help both the people affected and their doctors to build up a clearer picture of what may be causing the allergic reaction.
If, after testing, a diagnosis is unclear, or if it is suspected that an allergy may have naturally resolved, a process of medically supervised gradual feeding, or a food challenge, may be recommended. This may also be the case if it is thought that a person is only allergic to a food heated or cooked in a certain way.
To begin, the person will be given a very small amount of the suspected allergen. They will then be supervised for a period of 10 to 30 minutes. If no reaction occurs, another, slightly larger amount of the food will be given and they will be supervised once again. This process is usually continued for around 90 minutes. If any allergic reaction occurs, the challenge is immediately halted.
A food challenge should only be carried out under the supervision of medical professionals.
Home food allergy testing
There are a number of home allergy testing kits available on the market, including vega tests, kinesiology tests and hair tests. The science behind these tests is unproven, and they are unable to provide a definitive diagnosis. They should, therefore, be avoided and a doctor be consulted instead.
Treatment and management
The only completely effective way of managing a food allergy is to avoid the allergen completely. To do this, affected people should carefully check food packaging for the included ingredients. As a result of the Food Allergy Labeling and Consumer Protection Act of 2004, all food manufacturers in the US are required by law to declare, in easy-to-understand language, the presence of any potential allergens. 10
When eating out, extra care should be taken. Always inform restaurant staff of any allergy, and ensure that the chef responsible for your food is also made aware. The staff should be able to advise on which dishes are suitable and which are not. 10
In the event of an allergic reaction, there are two main types of medication available to relieve symptoms: antihistamines and adrenaline. 11
The symptoms of the majority of allergic reactions are caused by histamines. Antihistamines block the effects of the chemical, making them an effective treatment method, especially for milder allergic reactions. Many types of antihistamines are available at pharmacies without a prescription. Always be sure to read and follow the enclosed instructions.
Adrenaline is able to reverse the effects of a severe allergic reaction by opening the airways, reducing swelling, and maintaining heart function and blood pressure. 12
Older children and adults with allergies are often required to carry an adrenaline-containing auto-injector with them at all times. The auto-injector acts as the first line of response in the event of an allergic reaction. Additionally, the emergency services should always be contacted.
People required to carry an auto-injector will be taught how to use the device. Generally this is done by removing the cap, holding the injector firmly against the thigh and waiting for a clicking-sound that indicates that the injector has been activated.
People who are required to carry an adrenaline auto-injector should: 11
- Carry their device at all times.
- Be careful not to leave the device somewhere it may be exposed to extreme temperatures, such as a fridge or hot car. This can lessen the effectiveness of adrenaline.
- Regularly check the device’s expiry date.
- Use the device as soon as symptoms of an allergic reaction occur. Even if symptoms are mild, do not hesitate to use the auto-injector.
Glucocorticoids, a type of steroid hormone, are used to treat people experiencing anaphylaxis. They are usually administered intravenously after the affected person has been resuscitated in order to help relieve symptoms. 13 Despite common use, there is no consensus on whether glucocorticoids are effective.
Food allergy prevention
Although there are a number of proposals surrounding the prevention of food allergies, research into the area is still evolving.
The role of breastfeeding and the timing of first contact with potential allergens remains the central focus of most current thinking around prevention. A 2013 study by the American Academy of Pediatrics, for example, suggests that introducing solid foods to very young babies can trigger the onset of food allergies. Consequently, the report recommends breastfeeding for as long a as possible. 10
Another study, conducted by the National Institute for Allergy and Infectious Disease (NIAID), focussed specifically on peanut allergies. The study took a group of infants, all of whom were at a high-risk of developing a peanut allergy. It found that the children who were introduced to peanuts and regularly ate food containing peanuts before the age of 1 had an 81 percent lower chance of developing a peanut allergy than those who were not. 14
Despite this research, it is still advised that parents talk to a healthcare provider before taking any steps to try and prevent the onset of food allergies.
Food allergy FAQs
Q: Should I perform a food allergy test at home?
A: No, the use of home allergy testing kits is not recommended. The science behind these kits is dubious, and they are generally unreliable. People concerned that they may have a food allergy should visit a doctor for proper diagnosis.
- A flushed face
- Hives or a red rash around the eyes, mouth and tongue, which may spread to other parts of the body
- Swelling of the lips, the area around the eyes and the face
- Sneezing, watering eyes and a blocked or runny nose
- Nausea, vomiting and/or diarrhea
- An itchy or scratchy throat and mouth
More severe symptoms requiring urgent medical attention may include:
- Shortness of breath, wheezing or chest tightness
- Swelling of the tongue and/or throat, which may cause breathing difficulties
- A sharp drop in blood pressure, which may cause dizziness, confusion and loss of consciousness
A severe allergic reaction, or anaphylaxis, can be life-threatening. Urgent medical attention is required if one or more of these symptoms arises.
Q: Are allergic reactions to food immediate?
A: The speed at which the symptoms of an allergic reaction to food develop can vary. Reactions triggered by the release of the antibody immunoglobulin E (IgE) tend to come on fairly quickly, usually within a number of minutes to two hours. 1 Non-IgE-mediated allergic reactions, however, tend to come on more slowly. This can be anywhere between a couple of hours and a number of days after the allergen has been consumed. 17
Q: Are food allergy blood tests accurate?
A: Blood tests can be extremely useful to doctors and allergists trying to ascertain whether or not a person has a food allergy. However, they are not always accurate. In 50 to 60 percent of cases, blood tests will return a false-positive result. 18 This means, the results will show that a person has a food allergy, when in fact they do not. For this reason, a variety of diagnostic methods are likely to be used before a definitive diagnosis is given.