Introduction 

Monocytes are white blood cells (WBCs) that are derived from bone marrow. A monocyte is a component of the innate immune response and functions to regulate cellular homeostasis, especially in the setting of infection and inflammation.¹ Normal adult blood consists of these cells making up approximately 5% of circulating nucleated cells.² The optimal range for monocytes is 4 to 7 percent. 

Monocytopenia refers to a reduction in circulating monocytes, that is a common finding in myelodysplastic syndromes.³ Infection, trauma, medications, autoimmune disease, and some malignancies have an association with monocytosis, an increase in circulating monocytes, in the peripheral blood.¹  

Monocyte function 

Monocytes comprise a crucial component of the innate immune system.⁴ They are a type of WBC that differentiates into populations of macrophages and dendritic cells for regulating cellular homeostasis, specially in the setting of infection and inflammation.¹ Myeloid cells (a family of immune cells), consisting of neutrophils, monocytes, macrophages, and dendritic cells (DCs)—play key roles in anti-microbial defense and inflammation, as well as tissue repair and remodeling. Monocytes have toll-like receptors on their surfaces that interact with PAMPS (pathogen-associated molecular patterns) found on invading microbial cells.⁵ In response to such stimuli, there is migration of monocytes from the bone marrow into the blood circulation and infiltrate tissues within a period of 12 to 24 hours.⁶ 

Lifestyle and genetic factors affecting monocytes 

Although our knowledge concerning the development and function of monocytes and monocyte-derived cells during pathology (study of disease) is consistently expanding, it is also necessary to appreciate the behavior of these cells under healthy physiological conditions. However, healthy homeostasis is not a solitary state, instead several factors, including sex, diet, exercise, and age can come into play in order to influence the immune system. Whether there is a prevalence of genetic traits over environmental cues regarding the immune response remains a matter of debate.^7,8 

Environmental factors affecting monocyte levels 

Effects of air pollution on monocyte levels 

According to the World Health Organization (WHO), air pollution serves to be one of the most serious threats to our planet. In spite of a growing public awareness of the harmful effects that air pollution has on human health, the specific influence of particulate matter (PM) on human immune cells remains poorly understood. A study showed  the effect of PM on peripheral blood monocytes was investigated in-vitro.⁹ The analysis demonstrated a simultaneous PM-dependent activation of inflammasomes, including NLRP3 (nucleotide-binding oligomerization domain-like receptor pyrin domain containing 3) and Caspase-1, followed by the cleavage of Gasdermin D (GSDMD) and secretion of IL-1β. These observations suggest that the monocytes that were PM-treated, die by pyroptosis activated by two parallel signaling pathways, related to the inorganic and organic PM components. 

This report showed that PM exposure significantly impacts monocyte function and induces their death by pyroptosis. The observations indicated that the composition of PM plays a crucial role in this process.⁹  

Lifestyle factors that can have a positive effect on monocyte levels include:

• Avoiding injury and infection 
• Eating a well-balanced diet 
• Exercising regularly 
• Not smoking 
• Practicing good hygiene like washing your hands 
• Reducing stress 

Diet and nutrition 

Caloric restriction is known to improve inflammatory and autoimmune diseases. However, the mechanisms by which reduced caloric intake modulates inflammation are not understood completely. It has been demonstrated that short-term fasting results in reduction of monocyte metabolic and inflammatory activity accompanied with drastic reduction in the number of circulating monocytes. Regulation of peripheral monocyte numbers was observed to be dependent on dietary glucose and protein levels. Importantly, it has been shown that fasting improves chronic inflammatory diseases without compromisation of monocyte emergency mobilisation during acute infectious inflammation and tissue repair. Scientific literature has disclosed that caloric intake and liver energy sensors dictate the blood and tissue immune tone and link dietary habits to inflammatory disease outcome.¹⁰ 

However, overcoming nutritional deficiencies is apparently the most practical recommendation. Through the consumption of a balanced nutritious diet, the bodily requirement of optimal nutrition can be fulfilled that, in turn, has a significant impact on the immune system. It is not advisable to take supplementation of a single nutrient as food. Rather the incorporation of various fruits and vegetables, whole grains, proteins as well as probiotics may ensure adequate nutrient intake. Therefore, multi-nutrient supplements may benefit people having deficiency in spite of sufficient diet.¹¹ 

Increasing evidence has pointed towards the concept of excess salt consumption imposing risks on human health and a reduction in daily salt intake from the current average of approximately 12 g/d to 5–6 g/d has been suggested by public health authorities. The studies on mice have revealed that there is a role of sodium chloride in the modulation of the immune system, and tissue inflammation and autoimmune disease can be promoted by a high-salt diet. However, there is scarce translational evidence of dietary salt on human immunity. 

An experimental approach that fixed salt intake in healthy human subjects at 6, 9, and 12g/d for a period of months was undertaken in a study whereb the relationship between levels of salt-intake and alterations in the immune system was examined. The results showed that subjects having a high-salt diet of 12 g/d displayed a significantly higher number of monocytes compared with the same subjects on a lower-salt diet. Furthermore, a strong positive association was revealed between salt-intake levels and monocyte numbers. The decrease in salt intake resulted in reduced production of proinflammatory cytokines interleukin (IL)-6 and IL-23, along with enhanced production of anti-inflammatory cytokine IL-10. These results suggest that high-salt diet has a potential to bring about excessive immune response in healthy humans, which can be damaging to immune homeostasis, and a reduction in habitual dietary salt intake induces potentially beneficial immune alterations.¹² 

Impact of exercise on immune function 

Scientific evidence suggests that long-term physical activity (PA) is beneficial in improving the immune system and preventing the development of various infections. In terms of interaction with food intake, PA has not been shown to increase energy intake during a short-time course. Supplementation of probiotics, bovine colostrum, plant-derived products and functional foods, accompanied with PA, may provide additional benefits for improvement of the immune system.¹¹ 

Relationship between PA and immune system 

Meta-analyses have demonstrated that the risk of upper respiratory tract infections can be prevented by PA and, it also provides immunity, nevertheless supportive data are minimal and the results are inconclusive due to a small number of samples, leading to insignificant power, and unavailability of published studies. According to the meta-analyses, regular PA is associated with 31% lower risk of infectious disease and 37% lower risk of infectious disease-related mortality. 

Conversely, evidence is mounting that no PA and its consequences, such as adipose tissue accumulation, which leads to obesity, and muscle dysfunction, can negatively impact both innate and adaptive immunity. Further, it is now widely accepted that intensive PA leads to improvement in function of the immune system and that the recovery process is followed by beneficial adaptations. PA interventions of 3–5 times per week for an average of 30 minutes demonstrated higher CD4 T cells and salivary immunoglobulin IgA and lower levels of neutrophils. Neutrophils are the most abundant WBCs, the primary effectors of pathogen clearance, and the first WBCs recruited during infection. 

Furthermore, more research in relation to stress and recovery responses and exercise is needed to understand the beneficial effects of exercise on immune functions. This implies that people should be encouraged to engage in daily PA in order to strengthen their immune system and reduce the risk of infectious disease and infectious disease-related mortality.¹¹ 

Effect of smoking on monocyte levels

Cigarette smoking (CS)-related monocytosis leads to the development of chronic lung injuries via complex mechanisms. Researchers have aimed to determine correlations between measures of CS and monocytes, their capacities to predict chronic lung diseases, as well as their associations with mortality.

A single-center retrospective study of patients who underwent surgical resection for suspected lung nodules/masses was conducted. The analysis demonstrated that incidence of CS and greater monocyte numbers had an association with endpoints of chronic lung disease suggesting a contribution in pathogenesis.¹³

Emphasis on holistic approaches to maintaining immune health

Strengthening the immune system would involve the adoption of a holistic lifestyle approach encompassing balanced nutrition, good sleep, stress management, regular exercise, and incorporation of immune-supportive supplements. By nurturing our body and mind, we work towards fortifying our defenses, empowering ourselves to thrive in the face of challenges. 

Summary

Myeloid cells, consisting of neutrophils, monocytes, macrophages, and dendritic cells (DCs)play key roles in anti-microbial defence and inflammation, as well as tissue repair and remodelling. Monocytes are WBCs that are derived from bone marrow. The optimal range for monocytes is 4 to 7 percent. 

PM exposure significantly impacts monocyte function and induces their death by pyroptosis. It has been indicated that the composition of PM plays a crucial role in this process.  Caloric restriction is known to improve inflammatory and autoimmune diseases. short-term fasting results in reduction of monocyte metabolic and inflammatory activity accompanied with drastic reduction in the number of circulating monocytes. Regulation of peripheral monocyte numbers was observed to be dependent on dietary glucose and protein levels. Importantly, it has been shown that fasting improves chronic inflammatory diseases without compromisation of monocyte emergency mobilisation during acute infectious inflammation and tissue repair. 

Scientific literature has disclosed that caloric intake and liver energy sensors dictate the blood and tissue immune tone and link dietary habits to inflammatory disease outcome. Through the consumption of a balanced nutritious diet, the bodily requirement of optimal nutrition can be fulfilled that, in turn, has a significant impact on the immune system. Also, multi-nutrient supplements may benefit people having deficiency in spite of a sufficient diet. A high-salt diet has a potential to bring about excessive immune response in healthy humans, which can be damaging to immune homeostasis, and a reduction in habitual dietary salt intake induces potentially beneficial immune alterations. 

Scientific evidence suggests that long-term PA is beneficial in improving the immune system and preventing the development of various infections. 

CS-related monocytosis leads to the development of chronic lung injuries via complex mechanisms. 

Strengthening the immune system would involve the adoption of a holistic lifestyle approach encompassing balanced nutrition, good sleep, stress management, regular exercise, and incorporation of immune-supportive supplements.