Identifying immune biomarkers in healthy humans that indicate an increased susceptibility to upper respiratory tract illness (URTI) is necessary to develop improved diagnostic and treatment strategies. URTI is associated with substantial socio-economic and personal cost. Small to moderate reductions in the severity and duration of illness could lead to substantial reductions in these costs. This thesis investigated the relationship between the immune system and URTI in healthy individuals utilising exercise as a model of stress. Chapter 2 (Section 2.2) reviews the effects of exercise on the immune system and URTI, with a particular focus on the way in which exercise can be used to better understand the role of the salivary antimicrobial proteins (AMPs) lactoferrin and lysozyme in host defence. Determining mucosal immune status, that is the condition of the immune system at body surfaces interfacing with the external environment, is necessary to understand the role of the immune system in host defence. Exercise-related disturbances in the immune system may increase susceptibility to URTI, particularly when prolonged intense exercise is undertaken frequently. Th e link between exercise-induced disturbances in immunity and URTI risk suggests that exercise may be a useful model by which to study the relationship between immunity and illness in healthy individuals.