Around one in five people in the UK has hayfever, yet most of them have only a vague sense of why their immune system reacts so dramatically to something as ordinary as pollen. Understanding what is actually happening — and why it happens to some people and not others — makes the condition feel considerably less mysterious, and makes treatment decisions more informed.
Hayfever has a clear cause, a well-understood mechanism, and identifiable risk factors. None of it is random.
The Immediate Cause: Pollen
Hayfever is triggered by pollen — the microscopic particles released by plants as part of their reproductive cycle. Not all pollen causes hayfever with equal frequency. The most significant triggers in the UK are:
- Grass pollen. The most common cause of hayfever in the UK, affecting the majority of sufferers. Grass pollen season runs from approximately May through to August, peaking in June and July.
- Tree pollen. Begins earlier in the year — hazel and alder from January in mild years, birch from March through May. Birch is one of the most potent tree pollens and can cause severe symptoms in sensitised individuals.
- Weed pollen. Includes nettles, plantain, and mugwort, typically releasing from June through to September and extending the season for those sensitised to multiple pollen types.
- Mould spores. Technically not pollen, but airborne fungal spores that trigger allergic rhinitis through the same mechanism. They peak in late summer and autumn and are particularly prevalent in damp environments.
The pollen particle itself is not inherently harmful. The problem lies entirely in how a sensitised immune system interprets it.
The Underlying Mechanism: How the Immune System Gets It Wrong
Hayfever is classified as a type I hypersensitivity reaction — an overreaction of the immune system to a substance that poses no genuine threat. The process unfolds in two stages.
Sensitisation
The first time a person is exposed to a pollen allergen, the immune system may identify it as a threat and produce IgE antibodies specific to that pollen. This is the sensitisation phase, and it typically occurs without producing any symptoms — the person is not yet aware anything has happened. These IgE antibodies attach to mast cells, which are found throughout the body’s tissues, including the nasal lining, eyes, skin, and airways, effectively priming them to react on the next encounter.
The Allergic Response
On subsequent exposure to the same pollen, the allergen binds to the IgE antibodies on the primed mast cells. This triggers the mast cells to degranulate — releasing histamine, leukotrienes, prostaglandins, and other inflammatory mediators into the surrounding tissue. This release is rapid and produces the immediate symptoms of hayfever: sneezing, nasal discharge, itching, and watery eyes. A secondary, slower inflammatory wave follows hours later, driving the nasal congestion and fatigue that many sufferers find more persistent and disabling than the initial acute symptoms.
Histamine is the principal mediator driving most of the familiar hayfever symptoms, which is why antihistamines — drugs that block histamine receptors — are the most commonly used first-line treatment.
Why Do Some People Get Hayfever and Others Do Not?
This is the question most hayfever sufferers eventually ask. Everyone is exposed to pollen, but only some people’s immune systems misidentify it as a threat. The reasons are partly genetic and partly environmental.
Genetics and the Atopic Tendency
Hayfever is strongly hereditary. Having one parent with any atopic condition — hayfever, asthma, or eczema — approximately doubles the likelihood of developing one yourself. Having two atopic parents increases the risk further still. The genetic predisposition is not to hayfever specifically but to atopy — a tendency toward immune sensitisation to environmental allergens. This is why hayfever, asthma, and eczema so often cluster together in the same individuals and families.
The Hygiene Hypothesis
One of the most influential explanations for the rising prevalence of hayfever — which has increased substantially over recent decades — is the hygiene hypothesis. This proposes that reduced exposure to infections, parasites, and diverse microbial environments in early childhood leaves the immune system underemployed and prone to mounting inappropriate responses to harmless substances like pollen. Urbanisation, smaller family sizes, reduced outdoor exposure, and cleaner living environments are all associated with higher rates of allergic disease.
Cumulative Exposure and the Threshold Effect
Even in people with an atopic tendency, sensitisation requires sufficient cumulative exposure. This explains why hayfever often develops during childhood or young adulthood after years of pollen seasons, and why it can develop for the first time in adults who move to greener environments or spend more time outdoors. Think of sensitisation as a threshold — once crossed, the allergic response follows. The threshold varies between individuals and can shift over a lifetime.
What Makes Symptoms Worse?
Once sensitised, several factors influence how severe any individual’s symptoms are in a given season:
- Pollen count. Higher counts produce a greater allergen load and more intense symptoms. Warm, dry, windy days disperse pollen widely; cool, wet days suppress it.
- Multiple sensitisations. People sensitised to several different pollen types experience a longer and more continuous season than those reacting to a single allergen.
- Air pollution. Pollutants, particularly diesel particulates, increase the allergenicity of pollen particles and amplify the immune response. Urban hayfever is often more severe than rural hayfever for this reason, despite lower overall pollen counts in cities.
- Stress and illness. Both reduce immune regulation and can lower the threshold at which the allergic response is triggered, leading to more severe or prolonged symptoms in a given season.
From Understanding to Treatment
Knowing what causes hayfever — and why your immune system responds the way it does — is the foundation for choosing treatment that actually addresses the problem rather than just masking symptoms. At The Private GP in Birmingham, our doctors can assess your specific symptom pattern and pollen triggers, and discuss whether a hayfever and allergy injection or a more tailored treatment approach would give you better control this season.
Frequently Asked Questions
- Is hayfever caused by grass or trees?
Both can cause hayfever, and many sufferers are sensitised to more than one pollen type. Grass pollen is the most common cause of hayfever in the UK, with a season running from May through August. Tree pollens — particularly birch — affect a large number of people earlier in the year, from March through May. Knowing which pollen triggers your symptoms most helps with timing treatment and avoidance strategies.
- Is hayfever genetic?
Yes, strongly so. Having a parent with hayfever, asthma, or eczema significantly increases your likelihood of developing hayfever. The inherited predisposition is to atopy — a tendency toward allergic sensitisation — rather than to hayfever specifically. This is why all three conditions frequently appear in the same family, and why having one atopic condition increases the likelihood of developing another.
- Why is hayfever getting more common?
Hayfever prevalence has increased significantly over recent decades. The most widely accepted explanation is the hygiene hypothesis — reduced exposure to infections and diverse microbial environments in early childhood leaves the immune system prone to misdirected responses to harmless allergens. Rising temperatures from climate change are also extending the pollen season and increasing pollen production, exposing sensitised individuals to greater allergen loads for longer each year.
- Can you develop hayfever even if no one in your family has it?
Yes. While genetics plays a significant role, hayfever can develop without a family history of atopy. Environmental exposure, cumulative sensitisation over time, and individual immune variability all contribute. Someone with no family history can still cross the sensitisation threshold given sufficient exposure and the right environmental conditions.
- Is there a cure for hayfever?
There is no permanent cure, but hayfever can be very well managed with the right combination of treatments. Antihistamines and nasal corticosteroid sprays reduce symptoms effectively for most people. A hayfever injection provides sustained seasonal relief for those who need more than daily medication. Allergen immunotherapy — desensitisation treatment delivered over months or years — can reduce long-term sensitivity in selected patients, though it is a specialist referral pathway rather than a GP-level treatment.
Get the Right Treatment for Your Hayfever in Birmingham
Understanding what causes hayfever is the first step. Getting effective treatment is the next. At The Private GP in Birmingham, same-day appointments are available to assess your symptoms and discuss all available options — including the hayfever injection — so this season is meaningfully better than the last.