Resistance to infection of the external eye: the role of tears.

The external eye is continuously exposed to an environment containing potentially pathogenic microorganisms. One of the mechanisms which protects the eye from infection is the tear layer. We review the current knowledge of those antimicrobial substances known to be present in tears and the role they might play in preventing infection. These substances include lysozyme, lactoferrin, beta-lysin, and the antibody-complement system of proteins.