Allergy has now come to be known as a term defining altered reactivity to specific substances, which are otherwise harmless to people. The symptoms may include a dripping nose, itching eyes, sneezing, coughing, wheezing, hives, eczema, or anaphylaxis, and are caused by histamine and other chemical mediators that are released from mast cells or other cells in the body during a reaction between antigen and antibody.
Seasonal rhinitis is due to pollen from various plants.
Pollen is wind borne and the particles are light enough to be distributed by wind, particularly in dry and warm weather. Pollen from trees may include that from the maple, birch, or alder, to name but a few, or from different varieties of grasses and weeds. Patients with symptoms lasting all year might be allergic to dust mites, pets, mould, etc. Mould will be present outdoors in the air, especially on humid days following rain or melting snow. Indoor mould is often found in basements, bathrooms, or wherever humidity is high.