Hay fever is an allergic reaction to pollen. When these tiny particles come into contact with the cells that line your mouth, nose, eyes and throat, they irritate them and trigger an allergic reaction.

Allergic reaction

When you have an allergic reaction, your body overreacts to something it perceives as a threat. In hay fever, the allergen (the substance you are allergic to) is pollen. Your immune system (the body’s natural defence system) starts to respond as if it were being attacked by a virus.

Your immune system will release a number of chemicals designed to prevent the spread of what it wrongly perceives as an infection.

These chemicals then cause the symptoms of the allergic reaction, such as watering eyes and a runny nose.

Risk factors

It's unclear what causes the immune system to do this but there are several risk factors that can increase your risk of hay fever.

They include:

• having asthma or another allergic condition such as eczema (an allergic skin condition)
• having a family history of hay fever
• being exposed to tobacco smoke during early childhood

What are you allergic to?

In England, most people with hay fever are allergic to grass pollen. However, trees and weeds can also cause hay fever. Research suggests that pollution, such as cigarette smoke or car exhaust fumes, can make allergies worse.

Pollens

There are around 30 types of pollen that could cause your hay fever. The pollen that causes hay fever could come from sources including:

• grass – 90% of people in Britain with hay fever are allergic to grass pollen
• trees – about 25% of people in Britain with hay fever are allergic to pollen from trees, including oak, ash, cedar and birch (people with an allergy to birch often also experience an allergic reaction to apples, peaches, plums and cherries as these types of fruit contain a similar protein to birch pollen)
• weeds – such as dock, mugwort and nettles 

It's possible to be allergic to more than one type of pollen.

When is there most pollen?

Different trees and plants produce their pollen at different times of the year. Depending on which pollen you are allergic to, you may experience your hay fever symptoms at different times. In the UK, the pollen count season is usually separated into three periods:

• tree pollen – late March to mid-May
• grass pollen – mid-May to July
• weed pollen – end of June to September

However, the pollen count season can sometimes begin as early as January, or end in November.

The effect of the weather

The amount of sunshine, rain or wind affects how much pollen plants release and how much the pollen is spread around. On humid and windy days, pollen spreads easily. On rainy days, pollen may be cleared from the air, causing pollen levels to fall.

During their pollen season, plants release pollen early in the morning. As the day gets warmer and more flowers open, pollen levels rise. On sunny days, the pollen count is highest in the early evening.

Allergen

An allergen is a substance, such as pollen, that reacts with the body's immune system and causes an allergic reaction.

Allergy

Allergy is the term used to describe an adverse (bad) reaction that the body has to a particular substance.

Antibodies

An antibody is a protein that is produced by the body to neutralise or destroy disease-carrying organisms and toxins.

Immune system

The immune system is the body's defence system, which helps protect it from disease, bacteria and viruses.