Causes and Risk Factors of Food Allergies

A food allergy is an abnormal reaction to a food triggered by your immune system. Research from the Harvard School of Medicine suggests that anywhere from 6.5% to 8% of American children have a food allergy, while other studies have pegged the rate in adults at 10% or higher. What is confounding to those who suffer food allergies is why they have them and why certain foods are more likely to trigger symptoms than others.

To add further confusion, there are times when a food allergy may be considered a "true allergy," while others may be regarded as cross-reactive (meaning that the body reacts to what it thinks is the true allergy).

Biological Causes

At its heart, an allergy is a "case of mistaken identities" in which the immune system regards a harmless substance as harmful.

Under normal circumstances, your immune system will defend against infectious organisms and other invaders. It does so by recognizing substances on the surface of a cell known as antigens. The antigens it recognizes as harmful will trigger an immune response, part of which involves the release of defensive proteins called antibodies (also known as immunoglobulins). These antibodies will bind the antigen and trigger an inflammatory response to help neutralize the invader.

With an allergy, the immune system will overreact to an otherwise harmless antigen, referred to as an allergen. In launching a defense, the immune system will release a type of antibody known as immunoglobulin E (IgE). Among other things, IgE will spur the release of inflammatory chemicals, such as histamine, which cause the skin, lung, nasal, and throat symptoms we recognize as an allergy.

Each type of IgE the body produces has specific "radar" for each type of allergen. That is why some people may only be allergic to wheat, while others may be susceptible to multiple allergens.

In the United States, allergies to milk, eggs, wheat, fish, soy and peanuts are most common in children. In adults, fish, shellfish, peanuts, oleaginous fruits (nuts, seeds), and certain fruits (especially cherries, peaches, plums, apricots) are the predominant culprits.

Generally speaking, it is protein that acts as the primary food allergen. Such is the case with the proteins found in eggs, which can cause allergy when eaten, incorporated into other foods, or used to make vaccines.

Environmental Causes

The food traditions of a country will influence which foods the population will most likely be allergic to.

For example, in Northern Europe, cod is a staple of the regional diet and the most common cause of food allergy. In Italy, where vegetables comprise a higher proportion of the national diet, allergies to raw vegetables, tomatoes, and corn are common. The same can be seen in East Asia where rice is a staple food and the primary source of food allergies.

Common Food Triggers

Despite these regional variations, the World Health Organization (WHO) compiled a list consisting of eight foods that cause the most allergies globally, namely milk, eggs, peanuts, tree nuts, fish, shellfish, soy, and gluten-containing cereals.

In the United States, the U.S. Food Drug and Administration (FDA) has mandated that milk, eggs, peanuts, tree nuts, fish, crustacean shellfish, soybean, and wheat—which account for 90% of food allergies—be prominently listed on the product labels of any food containing them.

Of these, allergies to eggs, milk, wheat, peanuts, and soybean are more common in infants and young children, while older children and adults are more likely to be allergic to peanuts, tree nuts, and shellfish.

Sensitization

The reason why people are allergic to these or any other foods is not entirely clear. During the course of encountering a certain food, the immune system will decide whether to tolerate it or be sensitive to it. The latter process, called sensitization, is the mechanism that scientists still don't fully understand.

Some scientists believe that sensitization is caused by the way in which antigens are presented to the immune system. For example, when an unrecognized protein enters the body, it is captured by white blood cells, called dendritic cells, which carry them to the lymph nodes for inspection.

There, it encounters immune cells, called regulatory T-cells (Tregs), that regulate the immune response. At this stage, the Tregs will either tolerate the protein or activate the immune system and launch an attack.

With regards to food allergies, sensitization typically occurs in the digestive tract. While some researchers contend that food sensitization can occur via the skin, presumably as an extension of contact dermatitis, there is little actual evidence of this.

Atopy

Scientists don't believe Treg activation is simply a fluke. It may be that the Tregs are defective, but it also possible that a person has a predisposition to allergies, a condition referred to as atopy or atopic syndrome.

Atopy is believed to occur when a person is exposed to numerous allergens via the skin, respiratory tract, and digestive tract. These may include things like food, pollen, chemicals, pet dander, and dust mites. In some people, these exposures will leave the immune system in a heightened state of alert, placing them at risk of not one but multiple allergies.

Environmental Factors

But atopy doesn't entirely explain food allergies given that infants with little environmental exposure may also experience reactions. Within this population, it is the absence of an immune defense that characterizes the risk.

Scientists contend that children exposed to common food allergens will react because their immune systems don't yet recognize them. As their immune systems mature and the children gain exposure to a broader range of substances, their body will be better able to differentiate between harmless and harmful substances. But it is not always the case.

Recent research has shown that the early introduction of peanuts into a diet—ideally before the first birthday—can reduce the risk of a peanut allergy by nearly fourfold in later life.

As contradictory as this may seem, adult-onset food allergies are even more perplexing. Although little is known about the cause of the sudden onset, most scientists believe that sudden or severe changes in the environment may play a key role. Among the theories:

• Exposure to certain common bacteria or viruses (like the Epstein Barr virus linked to many autoimmune diseases) may alter the normal immune response.
• Exposure to a wider range of allergens than in years past, due to an age of increased continental and intercontinental travel, home relocations and/or international food transport.
• Pregnancy, menopause, and other sudden changes in hormones may also affect the immune response, as evidenced by higher rates of peanut allergy in women.
• Modern hygiene practices may spare children from exposure to microorganisms that build their immune defenses. Suppressed immune function in childhood may leave them vulnerable to allergies later in life.
• Food additives and/or methods of food processing alter the body's response to foods it might otherwise consider safe.

Others still insist that cross-reactivity—in which the presence of one allergy triggers others—is the primary cause of food allergies in adults.

Cross-Reactivity

Cross-reactivity describes an allergic reaction in which the body responds to one protein that is similar in structure to another protein. As such, the immune system will see both substances as the same. In the case of food allergies, examples of cross-reactivity include:

• Cow's milk and the milk from other mammals
• Cow's milk and soy
• Peanuts and tree nuts
• Peanuts, legumes, and soy
• Cow's milk and beef
• Eggs and chicken

There are other cases when a food allergy is secondary to a true allergy. One such example is latex-food syndrome in which an allergy to latex can cause an allergic response to food like avocado, banana, kiwi, or chestnut (each of which contains traces of the latex protein).

A latex allergy is considered a true allergy because having it increases your risk of a food allergy. By contrast, an avocado, banana, kiwi, or chestnut allergy is secondary because it does not increase your risk of a latex allergy.

A similar condition occurs with oral allergy syndrome (OAS) in which the presence of a true allergy can cause allergy symptoms when cross-reactive foods are eaten. With OAS, the true allergy usually involves tree or grass pollens but may also include aerosolized mold spores. Examples of OAS Include:

• Alder pollen and apples, cherries, peaches, pears, parsley, celery, almonds, or hazelnuts
• Birch pollen and plums, peaches, nectarines, apricots, cherry, tomatoes, peas, cilantro, almonds, or beans
• Grass pollen and melon, watermelon, oranges, tomatoes, potatoes, and peanuts
• Mugwort pollen and celery, carrots, dill, parsley, fennel, coriander, cumin, and sunflower seeds
• Ragweed pollen and watermelon, cantaloupe, honeydew, bananas, zucchini, cucumbers, and squash

Genetic Factors

Genetics play a significant role in the risk of food allergies. This is evidenced in part by the incidence of food allergies in families.

Similar familial links have been seen with seafood and other common food allergies.

To date, only a handful of suspect gene mutations has been identified. Most belong to a group of genes known as the human leukocyte antigen (HLA) complex. Among their many functions, HLA genes are responsible for encoding antigens on the surface of cells. Errors in the coding may explain why the immune system will suddenly see harmless cells as dangerous.

Other mutations involve the Filaggrin (FLG) gene which encodes proteins in skin cells, the Catenin Alpha 3 (CTNNA3) gene which encodes proteins in muscle cells, and the RNA Binding Fox-1 Homolog 1 (RBFOX1) gene which encodes proteins in nerve cells.

While this may suggest that genetic testing can be used to establish your risk of allergies, mutations on their own do not cause allergies.

Risk Factors

There is no way to accurately predict who will develop a food allergy, but there are certain factors that can place one at risk. Most of these are considered non-modifiable, meaning you cannot change them. Because allergies are influenced by genetics and ever-changing environmental dynamics, there are few, if any, individual factors that can reduce your personal risk.

From an epidemiological standpoint, there are six key factors associated with the development of a food allergy:

• Family history of food allergies
• Geography, namely where you live and the types of food common in the regional diet
• Age, with respect to the types of food allergies you will most likely develop at a certain age
• Sex, wherein certain allergies (like peanut allergies) are more common in women than men or vice versa
• Ethnicity, in which black children are more likely to have multiple food allergies than white children
• Other allergic conditions, including asthma, eczema, hay fever, or another food allergy

While changes in geography may remove you from the allergens that trigger your allergies, exposure to new substances (including those you eat, breathe, or touch) can potentially trigger new ones.