ABC of allergies: Summer hay fever

Environmental triggers

The main cause of hay fever in Britain is grass pollen, particularly perennial rye (Lolium perenne) and timothy grass (Phleum pratense). Symptoms peak during June and July. Symptoms in spring are commonly due to tree pollens, whereas symptoms in late summer and autumn may be due to weed pollens and mould spores. Rape seed may also provoke symptoms of rhinitis, although usually through irritant rather than allergic mechanisms. It has been suggested that emissions of nitrogen dioxide and ozone from vehicle exhausts have been increasing the sensitivity to airborne allergens.

Mechanisms of rhinitis

The symptoms of rhinitis are caused by an interaction between grass pollen and IgE on the surface of sensitised mucosal mast cells (type 1 hypersensitivity). The cells release mediators such as histamine and leukotrienes, which produce itch, sneeze, watery anterior nasal discharge, nasal congestion, and symptoms affecting the eyes.

Allergens are also recognised and processed by mucosal dendritic cells (Langerhans' cells) or macrophages, which then stimulate T lymphocytes to release interleukins, which promote tissue eosinophilia and IgE production. These compounds act to produce ongoing rhinitis, symptoms …