To date, a vast amount of research has aimed to explain how physical exercise alters the immune system. It is understood that stress induced by physical activity stimulates changes in the lymphatic system, but at present these changes are still not truly understood (2).
The lymphatic system is a one-directional circulatory system that serves to drain, filter and eliminate any forms of harmful substances or diseases from the body (4). It has the ability to neutralise invasive damaging agents such as pathogens. In a healthy individual, the properly functioning immune system comprises of lymphatic cells (humoral immunoglobulins) and additional cells outside of the lymphatic system. Correct functioning of these components identifies the proper physiological state of the immune system, and therefore a healthy organism (e.g. human) (2).
Immunoglobulins are a heterogeneous (diverse) group of proteins of the immune system composed by four polypeptides chains: two heavy chains (H) and two light chains (L) connected by disulfide bonds. There are five classifications (isotypes) of immunoglobulins based upon their structural differences in constants heavy chains. These five isotypes are: IgA, IgD, IgE, IgG, and IgM. The body produces two forms of IgA: serumIgA and secretory IgA. Saliva, tears and mucus are all examples of secretory IgA’s. Although it is believed that secretory IgA complements the neutralisation of harmful pathogens within mucus membranes and stimulates macrophage activation, the primary purpose of this immunoglobulin within serum is still not fully understood (5).
Nevertheless, due to salivary immunoglobulin A (S-IgA) dominance in the immune system of mucus membranes, it is typically considered as the first line of defence from environmental factors such as invasive pathogens. It is also believed that the concentration of S-IgA varies depending on the current physiological state and physical activity (2). Low concentrations of S-IgA have been linked with an increased risk of contracting URTI (6).
Amongst other factors, the secretion of S-IgA is stimulated by psychological and physical stress-levels induced by sport. The secretion and composition of S-IgA is controlled by the activity of the sympathetic and parasympathetic nervous systems. It is believed that stimulation of the autonomous nervous system (sympathetic and parasympathetic) can reduce the amount of saliva and/or inhibit its secretion – ultimately reducing the amount of S-IgA available (7). And as mentioned previously, a reduction in S-IgA is associated with an increased incidence of URTI (6). This information suggests that S-IgA may be a useful biological marker to distinguish athletes susceptible to URTI due to strenuous and/or excessive training (8). For this reason, sport scientists often monitor S-IgA amongst high-performance athletes.