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