Immune responses to intensified periods of training
Regular exercise training does not appear to appreciably alter blood leukocyte counts, including those of neutrophils (Gleeson and Bishop 2005). However, there are a few reports that exercise training reduces blood neutrophil counts in those with chronic inflammatory conditions or neutrophils in sites of chronic inflammation (Michishita et aL 2010), raising the possibility that such exercise acts in an anti-inflammatory fashion in those with elevated inflammation. This effect could be beneficial or deleterious, depending upon the context. While there is litde known about the influence of exercise training on neutrophil function, regular exercise, especially heavy, intense training, may attenuate neutrophil respiratory burst (Hack et aL 1994; Pyne et al. 1995). This could reflect a sustained effect of previous acute exercise, as attenuation of respiratory burst has been documented to last several days post-exercise (Suzuki et aL 1999). Both longitudinal exercise training and cross-sectional studies have shown that physically active persons exhibit reduced blood monocyte inflammatory responses to lipopolysaccharide, lower Toll-like receptor (TLR)-4 expression and a lower percentage ofCD14+/CD16+ 'inflammatory' monocytes (Gleeson et aL 2011a). There are two reports from the same group demonstrating an effect of exercise training on rat dendritic cells. Liao et aL (2006) reported that dendritic cell numbers increased after training, with no difference in co-stimulatory molecule (CD80 or CD86) expression, while Chiang et aL (2007) found that major histocompatibility complex (MHC) 11 expression, mixed leukocyte reaction and interleukin
Despite much research regarding the effects of exercise training on natural killer (NK) cell numbers and function, there appears to be much controversy regarding its effect. Early cross-sectional studies or interventions with limited subject numbers reported modest increases in NK cell cytolytic activity (NKCA) after moderate exercise training in previously sedentary subjects (McFarlin et aL, 2005; Nieman et aL, 1990, 1995; Peters et al., 1994; Shephard and Shek, 1999; Woods et al., 1999). In two of the larger trials, one study Fairey et al. (2005) found that 15 weeks of moderate exercise training increased NKCA compared with sedentary controls, while another 12-month trial found no change in NKCA in 115 post-menopausal women (Camp bell et aL 2008). However, periods of very intensive training have been shown to alter NK-cell subsets and to reduce NKCA (Gleeson et al. 1995b; Suzui et aL 2004; Morgado et aL 2012).