Food allergy develops when a person’s immune system becomes sensitized to proteins from a certain food. Whenever the person eats the allergenic food, the immune system launches an abnormal reaction as though the food were a dangerous invasive particle, such as a virus, bacterium, toxin, or parasite. Food allergies can cause a huge range of symptoms, which may be mild or severe. An allergenic food may cause a mild reaction in a person one day and a severe reaction in the same person the next day.

What Is a Food Allergy?

Our immune systems have many defenses designed to protect us against infections. Each defense has a different way of responding to attacks from foreign particles, or antigens. Antibodies (also called immunoglobulins or Igs), which are produced by white blood cells called B cells, are one of the body’s most important defenses.

Antibodies are proteins that can recognize specific antigens and launch an attack against them. The first time B cells come into contact with an antigen, they memorize the makeup of the particle. In the case of food allergies, the antigen is a protein from a particular food. On this first contact, there is no allergic reaction, but B cells begin producing antibodies called immunoglobulin E (IgE) specific to that antigen.

The IgE antibodies circulate through the blood and collect in the skin, lungs, and mucus membranes that line the nose, mouth, and throat. After this first contact, each time the antibodies come into contact with the antigen, they recognize it and launch an attack that involves triggering inflammatory chemicals — histamines, interleukins, and leukotrienes. The inflammatory chemicals flood the body, causing allergy symptoms.

The History of Food Allergies

Food allergies have been noted throughout history. Chinese emperors Shen Nong and Huang Di recorded their existence as early as 2,500 B.C. In ancient Greece, the philosopher Aristotle observed that some people experienced reactions to fruit that others did not. Ancient Greek physician Hippocrates noted that certain foods could cause death in some people.

Early in the 20th century, American pediatrician Oscar Menderson Schloss developed a skin test to diagnose food allergies. The test involved placing extracted protein from food on the skin to observe a reaction. However, this skin test can produce false positives — reactions that indicate a food allergy when the person is not actually allergic. Many doctors were frustrated by the lack of accuracy.

In the mid-20th century, scientists studied food allergens such as eggs, wheat, tree nuts, peanuts, and dairy. Aristotle’s centuries-old observations about fruit allergies were tested in a 1942 study. The results showed that some people have oral reactions to raw fruit, now known as oral allergy syndrome.

In the mid-1970s, American pediatrician Charles May developed the double-blind oral food challenge, which is considered the gold standard of food allergy diagnosis today. In a double-blind oral food challenge, a person receives increasing doses of the suspected food allergen and, at a separate time, a placebo (a harmless substance). Neither the doctor nor the patient knows which substance is given at which time, to avoid bias in the test results.

Epinephrine — also known as adrenaline — is a naturally occurring hormone which was first synthesized in 1904 by German chemist Friedrich Stolz. In the 1970s, American scientists Robert Lefkowitz and Brian Kobilka studied how epinephrine works in the body. They received the Nobel Prize in chemistry in 2012. Epinephrine is now used to treat anaphylaxis, a severe reaction to a food or other allergen. It is prescribed in a pen-like device that can be automatically injected into the muscle without needing to measure the medication or prepare a syringe.

Studies in the 1990s found that breastfeeding prevented atopic dermatitis and milk allergy in infants. This led to the belief that delaying an infant’s exposure to major food allergens (such as peanut, egg, and milk) would allow the immune system to “mature” and decrease the chance for food allergies later. Avoidance became standard advice from pediatricians and allergists. However, studies in 2015 and 2016 found that early oral introduction to peanuts dramatically reduced the occurrence of peanut allergy in children. Early introduction of eggs can also decrease the risk of an egg allergy. Early oral introduction has not yet been proven effective with other food allergens.

Today researchers are studying a type of oral desensitization called oral immunotherapy. Oral immunotherapy introduces minute amounts of allergens to allergic individuals to build up an immune tolerance. Several studies have shown a reduction in peanut allergy severity with this method. The U.S. Food and Drug Administration (FDA) has approved arachis hypogaea allergen powder (Palforzia) for peanut oral immunotherapy. Many board-certified allergists also prescribe oral immunotherapy for other foods, although these therapies are not yet FDA-approved.

How Common Are Food Allergies?

About 6 percent of children and adults in the U.S. have a food allergy. Food allergies are most common in children under age 3. Some children outgrow their food allergies. Less commonly, an adult develops an allergic reaction to a food they have eaten for years.

The prevalence of food allergies seems to be increasing, especially in Western countries such Australia, the United Kingdom, and the U.S. According to the Centers for Disease Control and Prevention (CDC), reported cases of food allergies in children under 18 grew by 18 percent from 1997 to 2007 in the U.S.

Which Foods Cause Allergies?

Nearly any food could cause an allergic reaction. However, nine foods are responsible for 90 percent of food allergies. The nine major food allergens and any ingredients derived from them are listed on food labels by law in the U.S. They are:

• Eggs
• Fish
• Milk
• Peanuts
• Sesame
• Shellfish
• Soybeans
• Tree nuts
• Wheat

What Are the Possible Symptoms of Food Allergies?

Symptoms of food allergy can vary widely. Some people develop skin symptoms such as itching, a rash, or hives. Others have respiratory symptoms such as asthma, sneezing, cough, and trouble breathing. Gastrointestinal symptoms including cramping, vomiting, and diarrhea are also common. Others experience anxiety, behavior changes, heart palpitations, dizziness, or many other reactions.

The most serious symptom of food allergy is anaphylaxis — a life-threatening reaction. Anaphylaxis is characterized by a sudden narrowing of the airways that makes it difficult to breathe, severe rash or swelling, or a sudden drop in blood pressure.

Learn more about food allergy symptoms.

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Have you or a loved one been diagnosed with food allergy? What allergens does your body react to? Share in the comments below, or start a conversation by posting on your Activities page.