Pollen triggers an immune overreaction in about one in four adults, causing symptoms that range from a runny nose to brain fog and poor sleep. Around 25% of U.S. adults have a diagnosed seasonal allergy, with women (29.5%) affected nearly ten percentage points more than men (20.7%), according to 2024 CDC data. Even if you’ve never been formally diagnosed, understanding what pollen actually does inside your body explains why some spring days leave you feeling wiped out.
How Pollen Triggers Your Immune System
Pollen grains are tiny protein packages released by trees, grasses, and weeds. When you inhale them, the grains land on the moist lining of your nasal passages and release their proteins, which diffuse across the tissue. In someone with allergies, the immune system treats those proteins as a threat and produces a specific type of antibody called IgE.
IgE antibodies attach to mast cells, which are immune cells stationed just beneath the surface of your nose, eyes, and airways. The next time pollen arrives, its proteins latch onto those waiting antibodies. This cross-linking causes the mast cells to burst open within seconds, dumping histamine and other inflammatory chemicals into the surrounding tissue. Histamine immediately increases blood flow and makes blood vessels leaky, which is why your nose swells shut and your eyes start watering almost instantly.
The Symptoms You Feel First
The initial wave of symptoms hits within minutes of exposure: itchy nose, mouth, eyes, and throat; sneezing; a runny nose; and watery eyes. Some people also notice reduced sense of smell right away.
A second wave often develops over the following hours. Nasal congestion sets in as inflammation builds, and you may notice clogged ears, a sore throat from post-nasal drip, coughing, headache, and puffiness or dark circles under your eyes. Fatigue and irritability are common too, and they’re not just from poor sleep. The inflammatory process itself can leave you feeling drained.
Effects on Sleep and Mental Sharpness
Pollen allergies do more than make your nose run. Swollen nasal passages increase airway resistance at night, disrupting sleep quality and causing daytime sleepiness. Research on children with seasonal allergies found that during pollen season, their reaction times slowed and their self-reported quality of life dropped. Kids with worse quality-of-life scores had measurably longer reaction times on simple movement tasks, and the link between allergies and poorer school performance has been documented in adolescents as well.
Interestingly, the cognitive slowdown doesn’t appear to be driven by stress hormones or inflammatory markers in the blood. Researchers found no correlation between reaction time and levels of cortisol, adrenaline, or key inflammatory proteins. The leading explanation is that disrupted sleep and constant nasal discomfort are enough on their own to impair focus and processing speed.
When Pollen Reaches Your Lungs
For some people, the reaction doesn’t stop at the nose. Pollen can trigger inflammation deeper in the airways, leading to mucus overproduction, tightening of the muscles around the bronchial tubes, and the wheezing and chest tightness characteristic of asthma. Animal studies confirm that pollen exposure in sensitized subjects produces inflammatory cell infiltration in lung tissue, excess mucus secretion, and heightened airway reactivity, all features that mirror human allergic asthma.
If you notice coughing, shortness of breath, or a whistling sound when you exhale during pollen season, that’s a sign pollen is affecting your lower airways, not just your nose.
Oral Allergy Syndrome: Pollen and Food
One of the more surprising things pollen does is change how your body reacts to certain foods. The proteins in some fruits, vegetables, and nuts look structurally similar to pollen proteins, so your immune system confuses them. This cross-reactivity is called oral allergy syndrome, and it causes itching or tingling in the mouth and throat when you eat the triggering food, usually only during or near pollen season.
The specific foods depend on which pollen you’re allergic to:
- Birch pollen: apples, pears, cherries, peaches, plums, kiwi, carrots, celery, hazelnuts, almonds, walnuts, peanuts
- Ragweed pollen: watermelon, cantaloupe, honeydew, bananas, zucchini, cucumbers
- Alder pollen: apples, cherries, peaches, pears, almonds, hazelnuts
- Mugwort pollen: celery, carrots, parsley, fennel, coriander, cumin
Cooking usually breaks down the offending proteins, so a raw apple might make your mouth itch while applesauce doesn’t.
What Happens if Allergies Go Untreated
Year after year of nasal inflammation can lead to complications beyond the typical sniffling. Chronic swelling of the sinus lining can block the openings that drain your sinuses, creating conditions where bacteria thrive. Sinusitis, the resulting sinus infection, is a common complication of allergic rhinitis. Repeated sinusitis can in turn trigger asthma flare-ups and promote the growth of nasal polyps, soft tissue growths that further block airflow. Polyps are especially common when sinusitis develops alongside allergic rhinitis.
When Pollen Counts Are Highest
Tree pollen season runs from roughly February through the end of May in temperate climates, with the worst stretch typically falling at the end of March into the first week of April before tapering off. Grass pollen peaks in late spring and early summer, while weed pollen (ragweed being the biggest offender) dominates late summer through fall.
Local pollen counts are measured in grains per cubic meter of air. For tree pollen, anything above 90 grains per cubic meter is considered high. Grass pollen hits the high range at just 20 grains per cubic meter, which is why grass season can feel brutal even when counts seem numerically low compared to tree season. Weed pollen is classified as high at 50 grains per cubic meter and above.
Reducing Your Exposure at Home
Pollen grains range in size but are generally well above the threshold that standard HEPA filters capture. A true HEPA filter removes at least 99.97% of particles down to 0.3 microns, which is smaller than any pollen grain. Running a HEPA-equipped air purifier indoors, especially in your bedroom, effectively scrubs pollen from the air you breathe while sleeping.
Other practical steps: keep windows closed during high-count days, shower and change clothes after spending time outside, and avoid drying laundry outdoors. Pollen counts tend to peak in the morning, so scheduling outdoor activities for late afternoon can reduce your exposure. Nasal saline rinses physically flush pollen from the nasal lining, which can reduce the amount of allergen available to trigger that mast cell chain reaction in the first place.