What causes a pollen allergy?
Short answer
Allergy question
A pollen allergy is caused by an overreaction of your immune system to pollen from trees, grasses, or weeds. Your body makes IgE antibodies against the pollen, and on later exposure those antibodies trigger cells to release histamine, producing the sneezing, congestion, and itchy eyes of hay fever.
This is a general research summary, not a diagnosis. A clinician or a test can confirm what applies to you.
Key facts
A pollen allergy is an IgE-mediated response in which the immune system reacts to inhaled pollen, and histamine released from mast cells drives the sneezing and runny nose.
Each spring, summer, and fall, trees, grasses, and weeds release tiny pollen grains into the air that trigger hay fever when inhaled.
Symptoms flare on warm, dry, windy days when pollen counts are highest and tend to ease after rain clears pollen from the air.
Here's the nuance
A one-liner rarely tells the whole story. This is the honest detail behind the short answer above.
A pollen allergy is caused by a case of mistaken identity in your immune system. Pollen is a fine powder released by trees, grasses, and weeds to fertilize other plants, and it is completely harmless to most people. In someone with a pollen allergy, the immune system incorrectly flags these tiny grains as a dangerous invader and mounts a defense against them.
The process begins with sensitization. The first time your immune system encounters a pollen it has decided to react to, it produces specific IgE (immunoglobulin E) antibodies against that pollen and attaches them to mast cells in the lining of your nose, eyes, and airways. You usually feel nothing at this stage. The next time you breathe in that pollen, it binds to the IgE antibodies, which signals the mast cells to release histamine and other inflammatory chemicals. Histamine is what actually drives the symptoms, causing sneezing, a runny or congested nose, and itchy, watery eyes, a pattern doctors call allergic rhinitis or hay fever.
Which pollens cause the reaction and when depends on the local plants. Tree pollen is the main trigger in spring, grass pollen in late spring and summer, and weed pollen, especially ragweed, in late summer and fall. Because pollen is carried on the wind, symptoms tend to flare on warm, dry, windy days and ease after rain washes pollen from the air.
Why some people develop pollen allergies and others do not comes down to a mix of genetics and environment. A family history of allergy, asthma, or eczema raises your risk, and repeated exposure over time is what allows the immune system to become sensitized in the first place.
The key points
The handful of things that decide the answer — skim these if you only read one section.
The immune system overreacts
Pollen is harmless, but in a person with the allergy the immune system misidentifies it as a threat and mounts a defense against it.
IgE antibodies drive it
On first exposure the body makes IgE antibodies to that pollen and attaches them to mast cells, a silent step called sensitization.
Histamine causes the symptoms
On later exposure the pollen binds those antibodies, mast cells release histamine, and that produces sneezing, congestion, and itchy, watery eyes.
Trees, grasses, and weeds by season
Tree pollen peaks in spring, grass pollen in summer, and weed pollen such as ragweed in late summer and fall, so timing points to the culprit.
Genetics and exposure both matter
A family history of allergy, asthma, or eczema plus repeated pollen exposure over time makes sensitization more likely.
A note on how to use this
Informational only — not a diagnosis
This page is a general research summary about allergies and allergy care. It cannot tell you whether you personally have an allergy or what is triggering your symptoms. It is informational only and never a substitute for evaluation by a licensed clinician.
If you want a real answer for your body, at-home allergy testing or a visit with a clinician can identify what you actually react to — this page cannot diagnose you.
❓Frequently Asked Questions
There is a strong genetic component. If one or both parents have allergies, asthma, or eczema, you are more likely to develop allergies yourself. However, genes only set the tendency; you also need repeated exposure to a pollen before your immune system becomes sensitized and reacts to it.
Yes. Pollen allergies most often start in childhood or the teen years, but they can appear at any age, including adulthood. Sensitization requires repeated exposure over time, so moving to a new region with different plants or a heavy pollen season can trigger allergies you never had before.
Pollen travels on the wind, so counts rise on warm, dry, windy days and drop after rain washes pollen from the air. Time of day matters too, with many pollens peaking in the morning. Higher pollen exposure means more histamine release and stronger sneezing, congestion, and itchy eyes.
Both cause a runny nose and sneezing, but a pollen allergy typically brings itchy, watery eyes and clear mucus, follows a seasonal pattern, and lasts as long as you are exposed. A cold is caused by a virus, often includes a mild fever or body aches, and usually clears within one to two weeks.
Reviewed by
Curex Editorial TeamReviewed for accuracy
Last reviewed August 2026
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