The term hay fever is a misnomer: the condition is not caused by hay, nor does it produce fever. The clinical name is seasonal allergic rhinitis, which is a type of allergy.

Seasonal allergic rhinitis occurs when airborne pollens or particles of plant or animal dander - small scales of hair or feathers, mould spores and so on - come into contact with the lining of the nose, eyes, or throat. In some people the immune system is overactive and identifies normally harmless particles as dangerous, producing an excessive reaction that causes inflammation.

This is known as allergy and the substances causing it are allergens. The result is a runny nose, itching and sneezing.

Hay fever is an allergic reaction to small particles of plant or animal protein that weigh little enough to be carried through the air and get deposited on the membranes of the eyes, nose and throat.

 
• Common sources of such particles are pollens of grasses, trees and weeds; mould spores; animal dander; and house dust.  
• When a plant or animal substance foreign to the human body invades it through the membranes of the eyes, nose or throat, an immune reaction occurs that is intended to counteract such invasion. Under ordinary circumstances this is a helpful, natural protection. However, some individuals exhibit an exaggerated inflammatory response to certain substances.  
• The allergens stimulate the body to form sensitising antibodies, which then combine with the allergens. The combination causes the body to release a number of chemicals that produce undesirable effects. Histamine is the best known of these chemicals. It causes swelling of the nasal membranes, itching, irritation and excess mucus production.  
• Early springtime hay fever is most often caused by pollens of common trees.  
• Late springtime pollens come mostly from grasses.  
• Colourful or fragrant flowering plants, such as many garden plants, rarely cause allergy, because their pollens are too heavy to be airborne. Unlike grasses, trees and weeds, which are wind-pollinated, garden plants are pollinated by insects, which carry their heavy, sticky pollen.  
• Pollinating seasons for grasses, trees and weeds are fairly consistent from year to year. These seasons vary according to geographic region. Hay fever season extends beyond the period of late summer into early autumn, as this is the time when many weeds release their pollen.  
• Allergens that are present throughout the year include animal danders, cosmetics, moulds, foods and house dust. House dust is a mixture of disintegrating cellulose (furniture stuffing), moulds, dander from pets, insect parts and small mites.  
• Mould spores also cause many allergy problems. Moulds are fungi that spoil bread, rot fruit and mildew clothing. They are present all year long and grow outdoors and indoors. Indoor plants and their soil, old books and damp places such as bathrooms, basements and laundry rooms are common sources of indoor mould growth. They also grow on dead leaves, grass, hay, straw, grains and on other plants and in the soil. Moulds are common in foods, such as cheese and fermented beverages.  
• People affected by hay fever may experience a number of symptoms. Most commonly these are sneezing, nasal discharge, congestion, itchy nose, itchy throat and watery, red and itchy eyes. Irritation and itchiness of the skin may also occur. Allergic rhinitis is often mistaken for the common cold, as many of the symptoms are similar.  
• Symptoms arising from house dust (including pet dander), foods, wool and various chemicals used around the house are frequently worse in winter, when doors and windows are kept closed.