False-colour scanning electron micrograph (SEM) showing 2 pollen grains of the ragweed, Ambrosia artemisiifolia, a causative agent (allergen) of hay fever (pollenosis). Hay fever is the commonest of all allergic conditions, occuring throughout the year in susceptible individuals when wind borne pollens or mould spores are inhaled. Persons suffering from hay fever produce a sensitising antibody - immunoglobulin E (IgE), which is present on the surface of cells in the nose & eyes. When this antibody encounters an allergen, chemicals (histamines) are released, producing swelling & inflammation of the linings of the nose, eyes & throat.