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Exposure to Acanthamoeba species appears to be ubiquitous, as 50% to 100% of healthy human subjects display anti-Acanthamoeba antibodies. However, the presence of specific anti-Acanthamoeba antibodies in the serum and tears of patients has not been investigated. The prevalence of serum immunoglobulin G (IgG) and tear IgA against three species of Acanthamoeba was assessed in healthy subjects and patients with Acanthamoeba keratitis.The level of specific serum IgG and tear IgA against A. castellanii, A. astronyxis, and A. culbertsoni in the sera of 23 patients and 25 healthy subjects was tested by enzyme-linked immunosorbent assays. Total serum IgM, IgG, and IgA concentrations were measured by nephelometry. Acanthamoeba keratitis was diagnosed clinically and confirmed by in vivo confocal microscopy. In some patients, corneal biopsies were also performed and trophozoites were cultured on lawns of Escherichia coli on non-nutrient agar.All healthy subjects and patients with Acanthamoeba keratitis had detectable serum IgG antibodies against all Acanthamoeba antigens. However, patients with Acanthamoeba keratitis had significantly higher anti-Acanthamoeba IgG antibody titers than healthy subjects. In contrast, Acanthamoeba-specific tear IgA was significantly lower in patients with Acanthamoeba keratitis in comparison with healthy subjects. Total serum immunoglobulins did not differ significantly between healthy subjects and patients with Acanthamoeba keratitis.The results suggest that a low level of anti-Acanthamoeba IgA antibody in the tears appears to be associated with Acanthamoeba keratitis.