Complex mechanisms are involved in mucosal defence at the air/epithelium interface. There are apparently two lines of protection.1 The first, which we are interested in here, prevents colonisation by micro-organisms on and through the epithelium. It is a non-inflammatory process, mainly mediated by (i) well known defence factors such as lysozyme and lactoferrin, (ii) specific secretory immunoglobulins A (sIgA), appropriate to surface protection, elicited by both local antigen administration and stimulation at related distal sites, and (iii) factors with cysteine proteinase inhibitor activity such as cystatin. Table 1 indicates the distribution of some of these proteins in several mucosal fluids.2-14 Together, these different factors keep potentially harmful micro-organisms and antigens out of the body. The second line of protection triggers an inflammatory response. This takes place when foreign material has penetrated the epithelium. It consists of a non-specific action of various systems and of a specific amplification of humoral and cellular immune response. Depending on the requirement of a particular body site, the composition of each secretion will be tuned appropriately to play numerous roles: wetting of the surface, exchange of gases, intake of nutrition and excretion of waste products, chemical and microbial defence, mechanical entrapment, and removal of potentially harmful micro-organisms and compounds.

The anterior part of the eye is protected by the mechanical action of blinking and the washing action of tears. The tear film maintains comfort and serves as the optical surface. Following the work of many researchers in this field, the classic concept of a three layered, aqueous dominated tear film15 has …