New twists to an old story: Novel concepts in the pathogenesis of allergic eye disease

The prevalence of allergy is rising globally at a very significant rate, which is currently at 20-40% of individuals in westernized nations. In the eye, allergic conditions can take on the acute form such as in seasonal and perennial allergic conjunctivitis, or a more severe and debilitating chronic form such as in vernal and atopic keratoconjunctivitis. Indeed, some key aspects of allergic eye disease pathophysiology are understood, such as the role of mast cells in the acute allergic reaction, and the contribution of eosinophils in late-onset and chronic allergy. However, recent developments in animal models and clinical studies have uncovered new and important roles for previously underappreciated players, including chemokine receptors on ocular surface dendritic cells such as CCR7, the contribution of conjunctival epithelium to immunity, histamine and leukotriene receptors on conjunctival goblet cells and a role for mast cells in late-onset manifestations. Furthermore, recent work in animal models has delineated the contribution of IL-4 in the increased incidence of corneal graft rejection in hosts with allergic conjunctivitis. Recent studies such as these mean that conventional paradigms and concepts should be revisited. The aim of this review is to highlight some of the most recent advances and insights on newly appreciated players in the pathogenesis of allergic eye disease.