During exercise, leucocytes are recruited to the blood, and if muscle damage occurs the cytokine level is enhanced. After prolonged, intense exercise the number of lymphocytes in the blood is reduced, and the function of natural killer cells is suppressed; furthermore, secretory immunity is impaired. During this time of immunodepression, often referred to as 'the open window', the host may be more susceptible to micro-organisms bypassing the first line of defence. This is of interest to top athletes who perform frequent severe exercise. Clinical observations regarding an increased risk of infections in top athletes are compatible with this model. However, in those performing regular moderate exercise the immune system will often be temporarily enhanced and this will protect these individuals from infections.