Since early studies investigated the influence of exercise on salivary secretory IgA (SIgA) in the 1980s, there has been demand for non-invasive biomarkers capable of monitoring the immune response to exercise, training and stress, and provide insight into whether such stressors may influence susceptibility to URTI. In spite of >30 years of research and ~200 original articles investigating a multitude of candidate markers, this tool remains elusive. Transmission of URTIs has been demonstrated via the nasal and ocular mucosae, so maintainence of a strong ‘first line of defence’ at mucosal surfaces is likely important for host defence. Tear fluid has been recently highlighted as a non-invasive medium for assessment of hydration status (through determination of tear osmolarity) and blood glucose concentrations (via glucose-sensing contact lenses). Prompted by the search for viable non-invasive immune biomarkers, this thesis set out to explore the potential of tear SIgA to assess immune status. First, in a prospective monitoring study, we demonstrated that tear SIgA secretion falls ~50% during the week before experiencing upper respiratory symptoms (URS), with a 30% reduction in tear SIgA secretion conferring a six-fold increased chance of experiencing URS in the following week. Next, we undertook three studies to explore the influence of everyday stressors on tear SIgA secretion. Both a two-hour bout of moderateintensity exercise and two-minutes of acute psychological stress caused an immediate ~50% decrease in tear SIgA concentration. The observations from the first study suggest that these reductions are of sufficient magnitude to temporarily compromise host defence, in line with the ‘open window’ theory. Dehydration, on the other hand, did not influence tear SIgA secretion. These studies provide the first experimental evidence that tear SIgA has potential as a non-invasive marker of mucosal immune competence that is sensitive to everyday stressors and has utility to assess common cold risk.