B-cells produce immunoglobulins that can help in the identification of infectious agents and fight against bacteria or viruses. Immunoglobulins, also known as antibodies, are glycoprotein molecules produced by plasma cells (effector B-cells). There exist five major antibody classes including: IgA, IgD, IgE, IgG and IgM. This classification reflects differences in the amino acid sequence in the constant region (Fc) of the antibody heavy chains. IgG and IgA are further grouped into subclasses (e.g., in human IgG1, IgG2, IgG3, IgG4, IgA1 and IgA2) based on additional small differences in the amino acid heavy chain sequences (15–18).

These glycoproteins act as a critical part of the immune response by specifically recognizing and binding to antigens, mostly to protein antigens but also to some carbohydrates, either infectious or not. Binding to antigens of bacteria, viruses or other pathogens may result in their destruction, as well as in other biological events. For instance, SIgA may bind to microbiota bacteria and do not directly destroy them but prevent their attachment to the epithelia (a process known as “immune exclusion”). This phenomenon consists in the agglutination of polyvalent antigens and their subsequent crosslinking, trapping within the mucus layer via the oligosaccharide chains of the immunoglobulins, and peristaltic clearing. Immunoglobulins may also help in antigen neutralization (for virus mostly), uptake for antigen presentation, and opsonization for phagocytosis.

IgG molecules possess heavy chains known as γ-chains, whereas IgM has μ-chains, IgA has α-chains, IgE has ϵ-chains, and IgD has δ-chains.

Free light-chains can be usually found in inflammatory and autoimmune conditions, often associated with ageing. They were primarily measured for diagnosing blood cancers such as myeloma, but now free light chains are thought to have direct anti-microbial action. Furthermore, recently, studies have shown that free light-chains may serve as a useful biomarker for monitoring the effectiveness of exercise interventions in healthy and clinical populations. Immunoglobulins can be measured in saliva and secretory immunoglobulin A or sIgA represents the most common form of IgA circulating in the human body (>85% of the total IgA levels). SIgA is secreted by B-lymphocyte cells in the lamina propria of all mucosal tissues, mostly in the intestine. For operational reasons, SIgA populating the salivary glands is sampled and measured. Its concentration tends to decrease with aging (19–21), due to a reduced salivary follow and secretion rate.

Cytokines constitute a large group of proteins, peptides or glycoproteins that are secreted by specific cells of the immune system and other secretory organs and contribute to cell signaling. The term cytokines may represent chemokines, interferons, interleukins, and lymphokines (22). These proteins are produced by several cells such as macrophages, B- and T-lymphocytes, endothelial cells, fibroblasts, and various stromal cells, including muscles. Myokines are signaling proteins produced by muscle cells that have been extensively implicated in aging (23–25). They have different effects on organs (liver, bone, pancreas) and exert various functions such as the regulation of muscle hypertrophy and myogenesis (26, 27) or the modulation of cellular stress (28).

Recently, cytokines and myokines have been proposed to belong to a superfamily of molecules termed as exerkines, which include also nucleic acids, peptides and proteins, and metabolites (29). Adipokines are also important molecules that are classified as exerkines (30) and they are critical in the elderly since adipocytes increase in number with ageing. Some authors (31–33) also include hepatokines (produced by the liver), osteocalcin and other bone secreted factor in the list of exerkines, emphasizing the complex crosstalk among different tissues and organs (muscle, bone, liver and adipose tissue) (34–36).

Concerning the effect of exercise, some studies have shown that acute bout of exercise, including incremental all-out treadmill exercises, may decrease salivary IgA in young athletes (37–46), whereas other studies have found no association between SIgA and exercise (47–62). Very few studies reported, instead, an increase of SIgA (63–66).

These inconsistent findings could depend on a variety of factors: lifestyle behaviors and different nutritional status of the studied populations (for instance, plasma glutamine concentration, which could mediate the levels of SIgA before and after exercise), the study design (cross-sectional versus longitudinal), technical variables including sample collection timing, sample transport, storage and pre-processing, and assay employed (enzyme immune assay, lateral flow assay, or point-of-care testing, which can vary in terms of reproducibility, reliability, sensitivity and specificity), sport-related variables (time between exercise and sample collection, type of training program, and type of exercise, in terms of intensity, modality and frequency), as well as individual variation, biological and circadian rhythms, and psychological factors (type of personality, and exposure to stressors, among others) (67).

Aging exerts subtle and complex effects on SIgA levels: mean SIgA levels tend to increase with age up to 60 years, and then slightly decrease (68). Concerning the effects of exercise among the elderly, Sakamoto and collaborators (69) recruited 92 community‐dwelling old women aged over 75 years, living in a rural area, who periodically performed approximately 20 min of low intensity exercise. In comparison with before exercise, saliva flow, SIgA concentration and secretion rate were significantly increased. Neves and coauthors (70) examined the acute effects of resistance exercise sessions performed at different intensities (50 versus 80% of one-repetition maximum) on SIgA absolute concentrations in a sample of 15 elderly women, aged 67.5 ± 3.9 years, performing two sets of 13 repetitions at 50% 1RM and two sets of eight repetitions at 80% 1RM. Resistance exercise sessions induced significant elevation in SIgA levels, compared to control session. Teixeira and coworkers (71) analyzed the influence of a 19-week exercise program on SIgA. Thirty-three subjects aged 68-95 years old participated and were distributed into 2 groups: 15 subjects performed aerobic endurance and lower and upper body strength exercise that included low-impact rhythmic work sequences with music, 3 times a week, and 18 remained sedentary. For the exercising group sIgA levels were higher after the 19-week exercise program with no changes for the control group.

These findings seem to suggest that exercise had mixed effects in youth, decreasing or not altering salivary IgA in young individuals, with few reports of increased values, while acute resistance and endurance exercise tended to increase salivary IgA levels in elderly people. Although the direct mechanisms underpinning exercise-induced changes in salivary IgA are unclear, neuroendocrine factors could be involved (e.g. cortisol) (72–74). In conclusion, SIgA is one of the most important mechanisms responsible for the defense against microbial invasion and health benefits that are associated with acute endurance and resistance exercise, which seems to counteract or at least partially mitigate the effects of aging.

Free light-chains are a major biomarker of malignancies, including autoimmune disorders and immunological impairments (75, 76).

Saliva free light-chains concentrations and secretion rates were measured in a sample of 88 young subjects, aged 18-36 years, and in a sample of 53 older adults, aged 60-80 years (75). While young adults completed a constant work-rate cycling exercise trial at 60% VO_2max or a 1 h cycling time trial, older adults completed an incremental sub-maximal treadmill walking exercise test to 75% HR_max. Saliva free light-chains levels were higher in older subjects: 3.91 [95%CI 0.75-19.65] mg/L and 1.00 [95%CI 0.02-4.50] mg/L in older adults versus 0.45 [95%CI 0.004- 3.45) mg/L and 0.30 [95%CI 0.08-1.54] mg/L in young adults, for kappa and lambda, respectively

A limited number of investigations has analyzed the effect of endurance training on SIgA in elderly individuals. For instance, active daily walking exercise training preserves mucosal immunity. Shimizu et al. (96, 97) recruited a sample of 51 males and 74 females, who took part into regular exercise sessions, 5 days a week for 6 months. Authors found a significant increase in SIgA level after training (p<0.05). Similarly, 30-minute active walking exercise training with an intensity of 80% ventilatory threshold for 12 weeks (98) led to elevated SIgA secretion rates in a sample of thirty elderly people, 8 men and 22 women, aged 66.7 ± 7.4 years (from 43.4 ± 28.4 to 66.8 ± 45.0 μg/min, p=0.003). In particular, stratifying according to the characteristics of the sample, the increase was marked both in participants who were less than 64-year-old (from 44.9 ± 35.0 to 70.9 ± 44.6 μg/min, p=0.021) and 65-85-year-old (42.8 ± 26.6 to 65.4 ± 46.0 μg/min, p=0.025) elderly subjects. The change was found in females (39.0 ± 29.6 to 59.0 ± 40.2 μg/min, p=0.015) whilst no statistically significant changes could be detected in male individuals (55.5 ± 22.3 μg/min at the baseline and 88.5 ± 53.0 after 3 months, p=0.104).

These findings were replicated by a 16-week home-based walking program, which was found to increase resting SIgA secretion rate (+37.4%; p<0.05) in 32 elderly healthy postmenopausal women. However, immediately after the acute bout of exercise, the secretion rate of SIgA and the saliva flow rate were significantly reduced (-32.3%, p<0.05, and -29.3%, p<0.05, respectively) (99). Dudhrajh (100) evaluated the impact of a 12-week group exercise program on salivary biomarkers of mucosal immunity in a sample of 95 elderly individuals aged between 60-86 years, recruited from five aged care facilities in South Africa. Subjects were allocated to two groups, which underwent a twice/week program (n=40) and a three times/week program (n=45). Increases in SIgA secretion rate were significant in both groups with small-to-moderate effect sizes (twice/week p=0.07, Cohen’s d=0.44; three times/week p=0.09, Cohen’s d=0.34).

Hwang et al. (101) studied a sample of 12 older women divided into a Pilates group (PG, n=6) and a control group (n=6). After the three-month Pilates exercise program, salivary flow rate was significantly higher in the PG than in the controls, in particular 30 min after acute high-intensity exercise.

All this, taken together, suggests that short-term endurance training may contribute to reinforcing the immune system in elderly individuals, by positively impacting mucosal immunity.

Exercise training can induce significant changes in cytokine secretion and signaling processes. Shinkai et al. (102, 103) reported that a higher concentration of IL-2 (p=0.021), IFN-gamma (p=0.015) and IL-4 (p=0.012) were found in a sample of 17 habitual male runners aged 63.8 ± 3.3 years compared to control group.

Rhind et al. (104) and Ogawa et al. (105, 106) also reported that elderly women who had consistently walked for one and a half hours once a week for 4 years had a higher level of IL-2 compared to untrained women.

Drela et al. (107) found that the effect of 24 months of aerobic exercise training practiced by older women (aged 62-86 years) could induce an increment of the production of IL-2, but it did not cause changes in the gene expression of IL-4 and IFN-γ.

Gueldner et al. (108) recruited a sample of 46 independently dwelling, ambulatory and mentally alert women aged 60-98 years, to explore the percentage of CD25 mitogen stimulated lymphocytes. Authors found a higher surface expression of IL-2 alpha chain receptor CD25 on T-cells from when stimulated with mitogen in vitro in the active group (n=25) versus the inactive group (n=21).

In another study of 22 male sedentary individuals aged 71.27 ± 0.82 years, Santos et al. (109) explored the effect of moderate exercise training consisting of running for 60 min/day, 3 days/week over 24 weeks at a work rate equivalent to their ventilatory aerobic threshold. Authors reported a significant decrease in TNF-α (43%) and IL-6 (37%) levels, an increase in IL-10 (27%) and non-significant changes in IL-1 and CRP. However, regarding the effect of endurance training in elderly patients, it has been shown that six months of endurance training may reduce the expression of TNF-α (from 1.9 ± 0.4 to 1.2 ± 0.3 relative U, p<0.05), IL-6 (from 71.3 ± 16.5 to 41.3± 8.8 relative U, p<0.05) and IL-1β (from 2.7 ± 1.1 to 1.4 ± 0.6 relative U, p=0.02) in skeletal muscle of chronic heart failure patients while systemic levels of these cytokines were unchanged (110).

Accordingly, 12 weeks of aerobic exercise reduced TNF-α concentrations (111, 112). Furthermore, 12 weeks of aerobic exercise training at 70–80% of individual maximal heart rate consisting of 45 min sessions of continuous aerobic exercise on a treadmill, stationary bicycle, arm bicycle, rowing machine, or a combination of these activities resulted in a significant reduction of all pro-inflammatory cytokines, CRP, IL-1, IL-6, and INF-gamma, as well as a significant increase in the anti-inflammatory, cytokine IL-10 in elderly (64 ± 7.1 years) coronary heart disease patients (113). Additionally, Lima et al. (114) reported that 10 weeks of endurance training reduce plasma IL-6 levels and maintain TNF-α concentration in hypertensive older adults.

In contrast, Pilch et al. (115) reported that regular endurance training (high-low) for 3 times a week over 12 weeks induces a significant decrease in the serum IL-1β (from 2.56 ± 0.3 to 1.17 ± 0.2 pg/ml, Δ=-1.39 ± 0.5 pg/ml, p<0.05) and an increase in the serum IL-6 (from 36.3 ± 5.9 to 45.8 ± 6.5 pg/ml, Δ=9.5 ± 2.5 pg/ml, p<0.05) in 15 non–smoking middle-aged women (42-47 years).

Finally, based on the previous studies, it seems that moderate- to high-intensity endurance training can reduce IL-6 and TNF-α, and increase IL-10 in elderly healthy and patient with chronic heart failure male individuals (116–121).

Few studies have focused on the impact of chronic resistance training on SIgA concentration among old subjects. For instance, Ahn and Kim (122) investigated the effect of a resistance exercise program using elastic bands (frequency of 3 times/week, 60 min/day, for 4 months) on sIgA level. Twenty-two elderly women were divided into an exercise group (77.91 ± 1.41 years) and a control group (78.73 ± 1.51 years). Levels of sIgA tended to decrease following the exercise program, even though not reaching the statistical threshold. Moderate-intensity resistance training over 12 weeks did not alter SIgA in sedentary low active elderly people (123). Thus, SIgA levels remained unchanged after 6 months of resistance training in older adults (124). Therefore, 12 weeks and 6 months of resistance training may not be enough to stimulate elevation in salivary SIgA levels.

The influence of exercise on circulating levels of cytokines has been described as decreased, elevated or unchanged. Many studies investigated the effect of resistance training on cytokines.

Rall et al. (116) found that this intervention has no effect on IL-1β, IL-2, IL-6 and TNF-α in older adult subjects. Accordingly, Bruunsgaard et al. (117) showed no significant changes in TNF-α, IL-6 and sTNFR-I after 12 weeks of resistance training in frail elderly people (86-95 years). Accordingly, 6 weeks of cycle ergometry reduced sTNFR-II concentrations and maintained TNF-α and IL-6 in elderly patient with chronic heart failure (118).

In contrast, 3 months of resistance exercise decreased both muscle TNF-α mRNA and protein levels in frail elderly individuals (119). Many physical activity parameters participated in variable results on responsive cytokines to exercise training such as intensity, duration, and type of exercise.

For instance, Calle and Fernandez (16) reported that resistance training alters cytokines responses dependant on exercise intensity and duration in men.

Evidence from epidemiologic studies in older adults reported that greater levels of physical fitness are associated with lower circulating levels of several inflammatory biomarkers, such as, IL-6, TNF-α, and CRP (120, 121).

Based on the available studies, it seems that short-volume (i.e., 6-12 weeks) resistance training did not alter IL-1β, IL-2, IL-6 and TNF-α concentrations in elderly healthy and patient with chronic heart failure, while 12 weeks of resistance training decreased muscle TNF-α mRNA in frail elderly individuals.

On the other hand , 12 weeks of strength training involved low-intensity resistance exercise decreased plasma concentrations of CRP, SAA but, maintained IL-6, TNF-α, MCP-1 levels, after the training program in healthy elderly women aged 85.0 ± 4.5 years (65, 66). Accordingly, 28 weeks of strength training can exert anti-inflammatory effects in older people, resulting into an increase in IL-10 levels occurring conjunctly with a slight decrease in the TNF- α/IL-10 ratio and maintenance of TNF-α levels in 33 older women with cognitive impairment, aged 82.7 ± 5.7 years (125).

In contrast, 10 weeks of strength training at eight-repetition maximum significantly reduced LPS–IL-6, LPS–IL-1β, LPS–TNF-α and circulating concentrations of TNF-α in elderly women aged 72 ± 6.1 years (126).

In summary, short-term moderate- to high-intensity strength training reduced LPS–IL-6, LPS–IL-1β, LPS–TNF-α and circulating concentrations of TNF-α, while, low-intensity strength training did not alter the above-mentioned biomarkers in healthy elderly women. Furthermore, strength training lasting 28 weeks increased IL-10 levels and slight maintained TNF-α levels in older women with cognitive impairment. It seems that strength training created an anti-inflammatory environment and better inflammatory balance in older people with cognitive impairment.

Tibana et al. (127) assessed the effect of 16 weeks of resistance training (three sets of 10 exercises, 6-12 repetitions maximum and 1-min and 30-s rest intervals between sets and exercises, respectively, 2 sessions per week) on irisin level in a sample of 49 older women with and without obesity aged 61-68 years. Circulating irisin decreased in the non-obese group compared with pre-intervention and obese group (p=0.01 and p=0.04, respectively).

To summarize, with a focus on randomized controlled studies, a recent systematic review and meta-analysis (128) has shown that resistance training can reduce CRP in older adults (standard mean difference or SMD=-0.61 [95%CI -0.83 to -0.31], p<0.001) and IL-6 (SMD=-0.19 [95%CI -0.42 to 0.02], p=0.07, statistically borderline significant), whilst no changes in TNF-α level could be detected. Moderators of CRP and TNF-α changes were found to be muscle mass, as well as a higher number of exercises (>8), a higher weekly frequency (3 times/week) and longer durations than 12 weeks.

Kukuljan (129), Peake et al. (130) and Dalla Via et al. (131) recruited a sample of 180 men aged 50-79 years who participated in an 18-month program of progressive resistance training plus weight-bearing impact exercise (3 day/week). Serum IL-6 decreased and was 29% lower [95%CI -62 to 0].