Role of histamine in allergic conjunctivitis.

Histamine remains the main mediator released by both specific allergic and non-specific mast cell activation. Histamine is the classic mediator of itching, flare and redness. The effects of histamine in allergic conjunctivitis are mediated by H1-receptor activation on blood vessels and nociceptive nerves. Histamine effects may be prolonged and exaggerated since a defect in the histaminase enzymes has been recently demonstrated in VKC. The effects of histamine on conjunctival tissues may be more complex than those manifested by the simple symptom of itching. In fact, proinflammatory effects of histamine on conjunctival epithelial cells and fibroblasts have been demonstrated. Preliminary results showed that the H1 antagonist, emedastine, reduces significantly cytokine production from histamine-stimulated fibroblasts.