What are some of the eye's natural defense that help to. prevent infection?

Tears: Tears contain antibodies, and there is an active flow from the tear glands across the eye to the drain of the tears (puncta). This flow helps keep the eye rinsed off, and replenishes the antibody rich tears. So tears serve many functions, including being an important barrier to infection. Additionally, the conjunctival and corneal epithelium serve as a physical barrier.

Many: Blinking, tears, the ocular surface system to name just a few. Not to mention the immune system.