Three major mechanisms have been reported to be involved in causing diseases included under the umbrella term of allergic conjunctivitis (or ocular allergy of the external eye surface):
a) The typical Type I hypersensitivity reaction, where the IgE mediated release of mast cell and basophil mediators is responsible for symptoms. (redness, chemosis, excess tearing and mucus, itching and burning), as a result of vasodilation, exudation, stimulation of glands and nerve endings.
b) Eosinophilic inflammation, both dependent and independent by a late-phase IgE mediated reaction.
c) Conjunctival hyperreactivity, often related to the eosinophilic inflammation but also possibly due to an abnormal tissue response to non-specific stimuli (cold air, pollutants, excess lighting,etc.).
These hallmarks of allergic eye disease, although often related to each other, depend on different genetic and environmental factors and may help to identify different phenotypes of ocular allergy with different clinical presentation, severity and treatment.