Flowers and trees are in bloom—and so are pollen allergies.
The sneezes, runny noses and itchy eyes that typically come with seasonal allergies are both miserable and extremely common. About one in four U.S. adults reported having seasonal allergies in 2021, according to the U.S. Centers for Disease Control and Prevention. And symptoms are getting worse and lasting longer: climate change is lengthening the growing season and exposing plants to higher carbon dioxide levels, causing them to produce more pollen. The tiny particles are not produced to hurt humans, so why do many of us react so poorly to them? It’s all an unfortunate consequence of our immune system’s attempts to navigate the world we live in.
“The immune system is very complex, and it has a pretty tough job,” says Mansi Kanuga, an allergist at Mayo Clinic Health System. “It needs to be able to recognize things that can be dangerous to us and know when to fight those things off, and it also needs to know when to settle down.”
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A human immune system has two parts: One mounts a general reaction to any foreign substance. Meanwhile the other responds to specific substances that the body has encountered before. The immune system will remember and develop dedicated resources to fight off those previous threats if the body runs into them again in the future. Allergies are such a target-specific immune response, and any substance that causes this kind of reaction is dubbed an allergen. “We can become allergic to any protein, really,” says Maya Jerath, an allergist at the Washington University School of Medicine in St. Louis and Barnes-Jewish Hospital.
How an allergy develops in the first place is still a bit of a mystery—but researchers have some solid theories. Scientists know babies aren’t born with specific allergies, Kanuga says, and they can come and go throughout someone’s life. Evidence also suggests genetics plays a role: children of parents with allergies are more likely to have them, too.
Allergy formation is also determined by how reactive the immune system happens to be during the first exposure to a substance. If the system is particularly reactive, it’s likely to deem the substance a threat to watch out for in the future. Different exposure routes can also affect the likelihood of developing allergies. For example, a person’s immune system is more accustomed to seeing new substances pass through the gut, so it is less likely to respond to such substances than it might be to compounds first encountered through the skin—which rarely lets in outside material.
But why is the immune system so markedly sensitive to pollen? After all, plants have dedicated defense mechanisms against herbivory, such as thorns and bitter-tasting chemicals, which don’t trigger seasonal allergies. Pollen is merely a substance plants use to reproduce—the botanical equivalent of sperm—and didn’t evolve to fend off humans. “You are not the target; they’re not trying to make the allergy,” says Nabarun Ghosh, a biologist at West Texas A&M University, who specializes in studying allergens.
Unfortunately, your immune system may not be willing to listen to reason about this. Pollen is difficult to acclimate to because it’s seasonal rather than consistently present in the environment. In addition, the tiny, airborne pollen grains that cause allergies can easily make their way past the nose and deeper into the respiratory tract. In general, however, allergies develop because the immune system is reactive when it first meets an allergen; the conditions don’t arise from any specific characteristic of the pollen itself.
Environmental allergies are mediated by a protein called immunoglobulin E, or IgE. Such an allergy’s initial development triggers the production of IgE molecules that are able to bind to the specific allergen. When the immune system detects the allergen again, it churns out IgE proteins, which bind to the surface of the body’s protective mast cells as scouts. When a scout IgE binds to the allergen, the mast cell releases a cocktail of chemicals—including histamines and other inflammatory substances that trigger the congestion, watery eyes and sneezing that we associate with allergies. An allergic response tends to remain strongest in the body part where the allergen was mainly encountered—so inhaling pollen might make your nose run, whereas getting it in your eyes might make them water.
While it might be tempting to hate on IgE and mast cells, this branch of the immune system has a long history of protecting people from parasitic infections. Modern humans don’t encounter these threats as often, so it’s easy to paint allergies as an overreaction.
Allergies are “your body’s immune system thinking that it’s doing the right thing,” says Emily Weis, an allergist at the University of Rochester Medical Center. “The immune system is always trying to differentiate between self and not self. That’s what has kept us alive.”
That’s cold comfort for allergy sufferers, of course. “We really recognize that seasonal allergies take a big toll on our quality of life for those of us who suffer with them,” Kanuga says. She, Weis and Jerath offer a three-pronged approach to living with allergies.
First, reduce exposure to known allergens when possible. This can include monitoring local pollen levels to determine when to spend time outside and leaving your windows closed to keep your house pollen-free. When you do have symptoms, antihistamines and other over-the-counter medicines can help manage itching, sneezing and watery eyes.
If these approaches aren’t sufficient, consider talking to an allergist about immunotherapy, which most commonly means allergy shots. These shots contain a personalized mixture of your allergens at low doses and act to desensitize your immune system, teaching it to tolerate these materials rather than unleash mast cells in their presence. In the U.S., immunotherapy is also available in tablet form for grass and ragweed pollens, as well as dust mites.
It does take time to see results: treatment with allergy shots can take about six months to improve symptoms and five years to complete, Jerath says. Still, she says, this approach is an impressive way to actually diminish allergies instead of just treating their symptoms. “It’s a little bit mind-blowing,” she adds, “to think about the fact that you can actually retrain the immune system.”