The outer surface of the eye is constantly exposed to a wide array of microorganisms. To protect the integrity or the ocular surface and to retain corneal transparency, a number of defense mechanisms have evolved. This article discusses the host mechanisms of the eyelids, tears, cornea and conjunctiva. These host defense mechanisms are identified as either a native, nonspecific defense or a specifically acquired immunological defense requiring previous exposure to an antigen and the development of specific immunity. Nonspecific components that protect the eye include the eyelids, ocular surface epithelium, normal flora and tear proteins. Specifically acquired immunity in tears, cornea and conjunctiva involves the interaction of antigen-presenting cells, lymphocytes and humoral components ofthe immune system.