Histamine is the main mediator of Type I allergic reactions. In ocular tissue, histamine release from mast cells induces itching, tearing, chemosis, oedema of the conjunctiva and eyelids, blood vessel dilation, and papillary reaction. Type IV hypersensitivity is a cell-mediated process involving T-cells, cytokines, and macrophage activation. The response peaks at 48 to 72 hours (called ‘delayed’ hypersensitivity) and results in tissue damage. (1)
Typical symptoms include sudden onset of ocular itching (the symptom of true itching is highly indicative of an allergy component), irritation, burning sensation and watering.