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The Science of Spring Allergies

For some, the arrival of spring means longer days, buds bursting into bloom, and a respite from the freezing temperatures and snowy days of winter before the blazing heat and humidity of summer. For others, it signals allergy season — a miserable few months of itchy eyes, runny noses, and constant sneezing. All are symptoms of allergic rhinitis, or hay fever, the condition caused by your body's immune response to common allergens such as pollen and dust.

According to the Centers for Disease Control and Prevention, 19.2 million Americans were diagnosed with hay fever in 2018. And the number of people affected may increase in the next few decades: A study published in 2021 suggests that climate change is lengthening pollen seasons in North America and making seasonal allergies worse. If you're one of the many who suffer each spring, you already know how disruptive they can be — but do you know what's actually happening in your body, or why it's happening? Here’s what you need to know about the science behind spring allergies.

How Your Immune System Handles Spring Allergies

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Normally, when your immune system detects a pathogen — such as bacteria, a virus, or another substance that can cause illness — specialized immune cells start making antibodies called immunoglobulins. There are several different types and classes of immunoglobulins, each of which attacks a specific pathogen to protect the body from infection. There’s a key difference between pathogens and allergens, though: Pathogens can affect anybody and always cause illness, while allergens affect only people who have sensitivities to them.

Pollen, for example, is not a pathogen, but in pollen allergy sufferers, the body treats it like one. When you have an allergy, the immune system recognizes harmless substances — for example, pollen, pet dander, or dust mites — as dangerous. If an allergen is inhaled or ingested, the system reacts exactly like it’s being threatened by a virus or parasite and starts making antibodies. Immunoglobulin E (IgE) is the main antibody linked to seasonal spring allergies. It travels to other parts of the body, triggering cells to release chemicals, such as histamine, against the allergen. Those chemicals are what cause seasonal allergy symptoms like a runny nose; watery, itchy eyes; sneezing; and congestion.

Why Seasonal Allergies Develop

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Allergies can affect anyone at any age — and, unfortunately, scientists don’t know exactly why they occur in some people but not others. According to the American Academy of Allergy, Asthma & Immunology, however, having family members with spring allergies is the most important factor in determining an individual's risk of suffering from them, too. Statistics indicate that if one of your parents has spring allergies, you have a 30% to 50% chance of developing them as well; if both parents have them, your risk increases to 60% to 80%.

Hormones, stress, and exposure to environmental irritants also may trigger allergies, or make them more severe. And people who never had allergies as kids may develop them for the first time later in life, even well into adulthood. But there is some good news: As mysteriously and suddenly as they appear, they can sometimes resolve themselves. (This is more common with food allergies than with seasonal allergies, but it's not out of the question.)

The Most Common Spring Allergens

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"Hay fever" is a misnomer, since tree pollen (not hay) is the biggest cause of spring allergies, and fever is not a symptom of them. Some North American tree species — including alders, ashes, birches, beeches, elms, oaks, poplars, and walnuts — are known to trigger more allergies than others. These species depend on wind to distribute their pollen (the male reproductive part of the trees), so their pollen grains are very light, powdery, and easily inhaled. This is also why, if you suffer from pollen allergies, you’re more likely to experience hay fever symptoms on dry, windy days. Conversely, rainy or humid weather prevents pollen from drifting, which lessens your chance of symptoms.

Organizations such as the National Allergy Bureau conduct pollen counts — measurements of the number of pollen grains in a cubic meter of air at a given place over a 24-hour period. The grains are usually captured with a sticky rod or trap and then taken to a lab to be counted. The higher the number, the more likely allergy sufferers in the area will have symptoms.

Trees that are pollinated by animals, such as fruit trees, basswoods, willows, and black locusts, typically don’t cause allergies because they don’t need to cast their pollen far and wide. Bees, bats, and butterflies do that job for them. Hay fever is less common in tropical regions because most tropical trees are insect-pollinated, while allergies flourish in temperate regions thanks to their abundance of wind-pollinated species.

Spring Allergies Vs. Other Seasonal Allergies

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Summer and fall allergies are caused by different plants, depending on when their reproductive season occurs. Grass and weed pollens, for example, are the most common culprit in summer. Allergy-triggering weeds such as English plantain can be found on practically any sidewalk or lawn in the U.S., while in agricultural regions, Bermuda grass and Timothy grass are widely cultivated to produce hay for livestock.

In the fall, ragweed causes the bulk of seasonal allergies. This incredibly productive pollen machine can turn out a billion grains per plant, which float on the wind for miles. It’s found in nearly every U.S. state in areas of disturbed soil such as roadsides, vacant lots, fields, lawns, and construction sites. While allergy sufferers may curse it, ragweed provides food for insects, songbirds, and small mammals.

Diagnosis and Treatment of Spring Allergies

If your runny nose and watery eyes occur at the same time every year, you might have a seasonal allergy. An allergist can make a diagnosis by applying tiny amounts of suspected allergens to your skin. If you have a strong reaction to one or more of them, you’re probably allergic to those substances. Your doctor may suggest taking an over-the-counter or prescription antihistamine, which blocks histamine’s ability to cause symptoms in response to the allergen. Another effective way to deal with seasonal allergies is by avoiding contact with allergens as much as possible. That includes checking your local pollen forecast, staying indoors with the windows closed on windy days, and taking a shower after you’ve been outside.