The tear film wets the front of the eye and is essential for producing crisp and clear images. It washes away foreign particles and dead cells and prevents the cornea from drying out.
The tear film is made up of three layers, each consisting of different components.
The outer layer is the lipid layer. Thanks to the high content of lipids (oil), it reduces evaporation from the underlying water (aqueous) layer. Its lubricating property also helps the eyelids spread the tear film evenly.
The middle layer is known as the water (aqueous) layer. As the name suggests, it is mostly comprised of water, in addition to large quantities of the proteins lysozyme and lactoferrin. These are part of the innate immune system when the eyes are open.
Lysozyme works like an antibacterial and destroys bacteria by damaging their cell walls and agglutinating bacteria.
Lactoferrin is a protein with very high iron-binding capabilities. Since bacteria need iron to grow, lactoferrin deprives bacteria of nutrients by absorbing iron.
The inner layer is the mucin layer. Mucins are gel-forming glycoproteins secreted by the goblet cells in the conjunctival epithelium. The mucin layer acts as a moistening agent for the naturally water-repellent surface of the cornea.