Why some people have allergies

Why do some people have allergies while others don’t? One theory suggests allergies are a relic of our evolutionary past. Our immune system, once primed to fight parasites, now mistakes harmless substances for threats.

Seasonal allergies don't always show up in childhood

The human immune system may become sensitized to pollen and develop an allergic reaction to it after prolonged exposure. Other triggers for new seasonal allergies may include exposure to new environments and changing climate conditions that affect allergy season length and conditions for various pollens.

Poison ivy, which produces urushiol throughout the plant, not just in the leaves, can cause year-round rashes.

Allergies occur when our immune system mistakenly considers a substance a threat

Immunoglobulin E antibodies, or IgE antibodies, may bind to an allergen, effectively labeling it as a threat. That triggers a cascade of actions in the body, including the release of histamine, which causes the characteristic itching and hives of allergies. Sometimes a cooked version of an allergen—like a cooked carrot—may look different enough from its raw form to the immune system that antibodies ignore it, and no allergic response takes place.

Zoo animals have developed allergies, too

Although scientists do not know if allergies manifest among wild animals—and if animals can survive with them—in a zoo setting, veterinarians have observed zoo animals with common allergy symptoms, including itchiness, crusty skin, and runny eyes. Veterinarians have identified Asian elephants with pollen allergies and at least one jaguar with an allergy to fleas.

The word allergy is partly derived from the Greek word 'allos' for 'other'

For people with allergies, their immune systems recognize substances as a harmful "other"—cuing an inflammatory response. This podcast explores allergy details and related topics, including how scientists are interested in if the same allergic response mechanisms could underpin other health issues, including severe, painful symptoms around the time of menstrual cycles. Since estrogen acts on white blood cells called mast cells, which leads to the release of histamines—similar to what happens during allergic responses—an understudied question is whether the same process is fueling some of the uncomfortable symptoms around menstruation. An allergist guest notes this raises questions about if antihistamine medications like Pepsid might alleviate them, something that hasn't yet been explored in clinical trials.

Poison ivy's rash-causing compound is found in the plant's roots, stem, and leaves—so coming into contact with any part of the plant can lead to rashes for the 85% of people who are allergic to it.

Poison ivy is in the same family as mangoes, cashews, and pistachios, and they all produce oily urushiol in their raw, unprocessed state—but they are typically prepared in ways to avoid causing human discomfort.

Amish kids have far fewer allergies than the general population

Only about 7% of Amish children in a northern Indiana community have tested positive for common allergens in routine skin prick tests, whereas half of the general US population tests positive for them. Asthma is also extremely uncommon among the Amish, even when compared with other farming communities. Researchers are interested in exploring if approaches like exposing kids across the general population to Amish farm dust might stave off some allergies.

Urushiol, the oily toxin in poison ivy, can remain allergenic on clothing for up to 5 years—or even longer.

Unlike food allergens, encountering inhaled allergens—like pollen—rarely causes life-threatening illness.

Tick bites can fuel an allergy to a sugar molecule in red meat

Alpha-gal syndrome is on the rise globally as ticks spread due to climate shifts. Bites from some ticks can trigger a sensitivity to a sugar molecule called alpha-gal found in mammal meat, which can lead to allergic responses to hamburgers and steak, among other products. Listen to an expert’s tips for avoiding this allergy, including quickly removing any ticks to limit exposure to their saliva or vomit.

Allergy tests identify antibodies, but not whether allergy symptoms may appear

Skin prick tests and blood tests, the two most common allergy tests, both detect the presence of allergen-specific antibodies. But just because the body has those antibodies doesn’t mean that a person is “allergic” to a substance or that they will develop symptoms of an allergy when encountering the substance. Allergists work with families to interpret the results and sometimes combine that data with approaches including exposure challenges—giving someone a tiny bit of the food or substance they may be allergic to—under medical supervision.

Modern conveniences may be contributing to allergy increases

A combination of genetics and how the environment changes our genes are likely contributing to the rise in allergies. But daily conveniences, such as dishwashers and packaged baby food, also change the level of bacterial exposures in early life, with potential implications for allergy development, experts from Mount Sinai Health System state in this video discussion.

In 1982, around 20% of the people who ate some of the roughly 7,400 bags of cashews sold as part of a Little League fundraiser in Pennsylvania broke out in rashes because of natural urushiol residue on the nuts' shells that hadn't been removed and fully cooked off—the same allergy-inducing substance found in poison ivy.

Wichita is the worst US city to live in if you suffer from seasonal allergies

More than 100 million people in the United States suffer from allergies each year. But Wichita, Kansas, tops the charts year after year as the worst city in America for those suffering from seasonal allergies. Longer and more intense pollen allergy seasons, however, are increasing the burden of living with seasonal allergies in numerous locations across the country.

See if you know how to recognize poison ivy with this quiz.

Itching is caused by stimulated nerve cells in the skin

Scratching produces low-level pain signals to the brain, distracting from the itch. Scratching also releases serotonin, a neurotransmitter that creates a happy sensation, which resets the itch signal. This can cause a cyclical itch-scratch cycle.

The science of food allergies

Why do certain substances trigger an immune response while others don't? Listen to researchers explain the science in this five-minute video. Food allergies affect around 8% of children and 2% of adults in the US, and—unlike sensitivity or intolerance—they involve the immune system mistaking certain harmless proteins in foods as dangerous invaders, triggering a rapid and sometimes severe defense response.

Inhalers, antihistamines, immunotherapy, and more have made allergies a multimillion-dollar market

Market research suggests that the allergies market will continue to grow in the years ahead as the number of allergy diagnoses increases, identifying new patients. Areas of growth include the increasing interest in immunotherapies designed to retrain the immune system through regular exposure to potential allergens via injections and under-the-tongue medical formulations, among other approaches.

Poison ivy is projected to be larger and more resilient due to climate change.

Between 10 million and 50 million people in the US develop rashes from poison ivy, poison sumac, and poison oak annually.

More than 90% of America's food allergies are triggered by 9 foods.

Alpha-gal allergy is a problem for red meat lovers, and that same alpha-gal sugar is an issue for xenotransplants, too—since it's present in pigs, human bodies reject pig-to-human transplants as foreign once they detect the substance.

Embracing poor hygiene in early life won't stave off allergies

The hygiene hypothesis, first proposed in 1989 by British immunologist David Strachan, suggests that early life exposure to microbes—like those from farm dust or pets—properly trains our immune systems to respond to threats. But sometimes the idea is misinterpreted: It doesn’t mean that poor hygiene—like failing to wash hands–will help ward off allergies; that type of behavior leads to infections and won’t help with allergy prevention.

Adults are developing food allergies that they didn’t have as children

Research published in 2019 found that almost half of 40,000 surveyed adults with food allergies developed a new food allergy as adults, underscoring that our immune systems may newly misclassify substances later in life.

Kids often grow out of allergies to milk and eggs

Between 60% and 80% of kids with allergies to milk or eggs outgrow them by age 16. The immune system doesn’t always recognize and respond to baked forms of these allergens, so when kids eat muffins and other cooked food products, their immune system may become more habituated to them and overcome food sensitivities. Aging out of other allergies is less common.

Long-term goose bumps are something to flag to your doctor

Although rarely a cause for concern, persistent goose bumps with no known cause could be a symptom of something to discuss with your doctor, including anxiety, a hair follicle blockage, or conditions that arise from an overreaction of the autonomic nervous system.

Gut microbiome rewrites may help reduce allergies

Researchers are investigating different strategies to determine if one day they could give people a combination of gut microbes to set them up for fewer allergies. In animal research, scientists have already prevented severe allergic responses in allergy-prone mice—supplying them with gut microbes from healthy, non-allergic human babies.

Autoimmune disease prevalence and incidence are on the rise—but no one knows why

Researchers theorize that environmental factors, including shifts to less healthy diets, air pollution, infections, stress, climate change, and exposures to xenobiotics—substances like pesticides, plastics, and toxins that incite immune responses—may all be fueling epigenetic changes to our bodies that make people's immune responses go awry.

Poison ivy's characteristic rash is caused by an allergic response to an oily substance in the plant called urushiol.

Home allergy tests don’t work well and experts urge caution

Doctors do not recommend at-home allergy test kits because their results may be inaccurate and confusing, and lead people to make lifestyle changes that are not warranted. Tests that require sending in a hair sample, for example, will not be helpful because hair does not have the relevant antibodies that blood testing would detect.

Most people breathe fungal spores daily without getting sick

Most people breathe in hundreds of Aspergillus fumigatus spores daily without getting sick—it’s a common microscopic fungus in our environment. But people who are severely ill, including those who have undergone an organ transplant, are at risk of developing infections from these spores.

Eczema appears to heighten the risk of developing shingles.

CRISPR technology is removing allergens from common foods

Proteins responsible for allergies to eggs, milk, wheat, peanuts, and other foods can be genetically edited out of foods to reduce the risk of life-threatening reactions. The technique has also been used to produce caffeine-free coffee beans, spicier tomatoes, allergen-free pets, and more nutritious fish.

Most of the world’s population has difficulty processing lactose—but that’s not an allergy

Lactose intolerance is a digestive condition that occurs when the small intestine has difficulty breaking down lactose, a sugar found in products like milk or cheese, leading to symptoms such as diarrhea or bloating. Almost 70% of the world’s population may have lactose processing issues, particularly in Asia and Africa. This is different from a condition called cow's milk protein allergy—an immune system disorder that can cause hives and anaphylaxis.

Scientists testing how climate change affected plants found in a yearslong simulation study that poison ivy grew 70% faster than it did without extra carbon dioxide.

Black mold isn’t more worrisome than other colors of mold

Black mold typically refers to Stachybotrys chartarum, a mold that commonly appears in damp or water-damaged areas of the home. Black mold—and other molds—won’t typically make otherwise healthy people ill, and all molds should be treated the same with respect to health risks and removal. If you have mold allergies, mold can cause an allergic response, and it can trigger asthma attacks for people with asthma. If you have a compromised immune system, then black mold exposure can also cause a fungal infection in your airways or elsewhere in the body.