Hay fever, not caused by hay!

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.