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Allergy misery has a high sticker price. Allergies are the sixth leading cause of chronic illness in the United States, with an annual cost in excess of $18 billion, according to the Centers for Disease Control and Prevention (CDC). Then there are indirect costs, which include lost workdays, missed school time, and curtailed leisure activities. And the situation is getting worse: in Western industrialized countries and heavily urbanized areas of other countries, allergies—the body’s overly sensitive immune response to a harmless substance—are on the rise. Furthermore, scientists believe that the ramifications of climate change for allergic disease are likely to be significant.

Allergies can range from irritating inconveniences to chronic debilitating conditions. They can even be life-threatening, as in the case of anaphylaxis, which is a whole-body allergic reaction. Because of their potential severity and increasing prevalence, allergic reactions have been the focus of rigorous research. This work has resulted in a greater understanding of the complex nature of allergic reactions, which in turn has led to more effective treatment options. Doctors are still learning more about how to manage symptoms successfully.

Five major allergens

Although there are many substances (allergens) that trigger allergic reactions in humans, these five categories account for the majority of cases:

  1. Pollen - Allergies that come and go with the season are usually triggered by plant pollen.
  2. Insects - Cockroachs and dust mites can cause allergies.
  3. Pet dander - Lovable but sneeze-inducing cats and other furry pets are a common cause of allergies.
  4. Mold spores - A hidden source of mold spores is the damp soil of house plants.
  5. Foods - Peanuts are a common trigger for food allergies.

Why are you allergic?

Allergic reactions are inappropriate, overblown responses mounted by the body’s immune system against a harmless substance. Take ragweed pollen, for example. Ragweed pollen is not poisonous, infectious, or in any way harmful to humans. But in some people, it triggers an attack by the immune system—an allergic reaction. When this happens, the ragweed pollen (or any other offending substance capable of triggering an allergic response) is called an allergen.

You can develop allergies at any point in your life. Allergies typically develop for two reasons. First, you may be genetically predisposed to be allergic. Second, factors in your environment, especially when you are young, may make you more susceptible. Most allergies are caused by some combination of these genetic and environmental influences.

Your genes

While many people suffer from allergies, most don’t. In the United States, one person in five is an allergy sufferer. To some extent, being allergic runs in families. For instance, a child with one parent who has allergies has a 50% risk of developing allergies. This risk increases to 70% if both the child’s parents are allergy sufferers.

Someone with a genetic predisposition to allergies is said to be atopic and more likely to suffer from childhood allergies, which are also known as atopic diseases. People who are atopic are typically afflicted with one or more types of allergy throughout their lives.

Eczema (atopic dermatitis), one of the most common atopic diseases, typically first appears in very young children with eczema’s signature itchy, red rash. As children with eczema grow older, they are more likely to develop the symptoms of nasal allergies or allergic rhinitis—sneezing, runny nose, and congestion. And many then go on to develop the lung symptoms associated with allergic asthma by age 5 or 6. Also, about one-third of children with moderate to severe eczema develop food allergies.

Allergies and climate change

Most scientists believe that human-influenced climate change is already under way. Climate change may bring many unfortunate ramifications, including a likely increase in allergic diseases.

In a study of climate change and allergic disease, researchers investigated how changes in temperature would lengthen growing seasons and which trees and plants would benefit. They concluded that the resulting amount of airborne allergens would increase, leading to a higher burden of seasonal allergies.

This process has already begun. For example, research has shown that the climate change that has already taken place enables a ragweed plant of today to produce significantly more pollen per plant than its predecessor of 100 years ago and for a longer period of time each year. A 2011 study found that in parts of North America, the ragweed pollen season had increased by two to four weeks since 1995. That’s bad news for people with fall allergies.

Your environment

Genes alone are not enough to cause allergies. The circumstances of your early childhood influence how likely you are to develop allergies. For instance, if you have siblings, your place in the birth order matters. Firstborn children are more likely to suffer from food allergies than their younger siblings. Scientists think this is because younger siblings are exposed to more germs passed around by older brothers and sisters. Exposure to a wider array of germs early in life may dampen the body’s tendency to turn on the allergic response. Similarly, children in day care, who are exposed to germs as they come in contact with many other children, seem less likely to develop asthma.

It was once thought that breastfeeding gave a child some protection against developing allergies, but this theory is now controversial. Studying the effects of breastfeeding is difficult, because families who choose to breastfeed are often different from families who don’t in many other ways besides this one choice. To date, there is some evidence that exclusive breastfeeding in the first few months of life may reduce the likelihood that an infant develops eczema, but it has not been clearly shown that breastfeeding has a long-term impact on the risk of environmental allergies, food allergies, or asthma.

The hygiene hypothesis

Since the early 1990s, evidence from around the world has supported the notion that the fewer microbes you encounter early in life, the greater your chance of developing allergies. This theory, commonly referred to as “the hygiene hypothesis,” is a proposed explanation for the development of all types of allergies. Proponents of the hygiene hypothesis point to evidence that exposure to microbes helps “train” the developing immune system by stimulating T cells that dampen an allergic reaction. Without sufficient exposure to the bad guys in early childhood, certain components of the developing immune system don’t develop properly and overreact to harmless things.

Researchers suggest that modern plumbing, cleaner homes, cleaner food and water, and the introduction of antibiotics and vaccines are in part culpable. Such innovations, which have helped lower the rate of infectious diseases and childhood death, also have reduced the number of microbes children encounter. Their contemporaries in less developed countries have a higher rate of infectious diseases, but a lower rate of allergies.

For instance, researchers observed that after the reunification of East and West Germany, seasonal hay fever and asthma were less common in children who had spent their early years in less affluent East Germany. After the rise in East Germany’s standard of living following reunification, hay fever increased among East German children. Even today, rates of allergic diseases are high in Western industrialized countries but not in other regions, such as rural areas of Africa and Asia.

Being around animals, especially farm animals, is also protective. Environments with these animals have higher levels of bacteria. Researchers who studied young children living on farms proposed that they are less likely to develop allergies than those raised in urban settings because their immune systems received strong stimulation in infancy from bacteria. Other studies have suggested that having a dog, cat, or other furry creature in the house during early childhood also lowers the risk of allergy, perhaps because of the microbes those pets carry.

So, what should we do? Experts absolutely do not recommend that parents purposefully expose their children to germs or avoid getting a child vaccinated to prevent allergies; vaccines are still vitally important. But a sterile environment may not always be necessary. When infants and toddlers put their “dirty” fingers and other objects into their mouths, not only are they learning about the shape and nature of those objects, but also their immune system may be learning about the outside world. Likewise, although all children should have their hands cleaned before eating, it is not necessary to pull out the hand sanitizer each time a child has been crawling on the floor.

Two types of immune defense

The hygiene hypothesis posits that insufficient exposure to microbes early in life fails to “train” the immune system to distinguish between disease-causing germs and harmless allergens. The immune system has two lines of defense.

The innate immune system

Also known as the nonspecific immune system, the innate immune system provides the type of immunity everyone is born with. It is the body’s first line of response to germs or allergens. Though the innate immune system doesn’t have antibodies to specific germs, it triggers a swift inflammatory response to anything it identifies as a “non-self” invader. Response time is typically within minutes to hours.

The adaptive immune system

The second line of defense, the adaptive immune system targets specific germs and allergens and “remembers” which ones it has encountered before. The adaptive immune system requires “schooling”, however, so it can recognize these pathogens when it meets them again. During an invasion, it takes longer than the innate system to become activated—but once it is, it is immensely powerful.

Genetic modification debate

There is some discussion about the role that genetically modified (GM or GMO) food might play in the escalating incidence of food allergy, food intolerance, and digestive illnesses. Genetically modified food is the result of scientists inserting DNA from another species into the plant—typically corn, soybeans, canola, cotton (the source of cottonseed oil), and sugar beets—to make the plant resistant to insect damage, viral infections, and certain herbicides.

The FDA says that the GMO foods it has evaluated through its voluntary consultation process are not more likely to cause an allergic or toxic reaction than foods from traditionally bred plants.

There is a lack of long-term independent scientific studies published in peer-reviewed journals that examine how ingesting GMO foods affects humans.