Polysaccharides, such as the plant polysaccharide in Astragalus membranaceus, Longan, and L. barbarum (Yin et al., 2019), and the fungal polysaccharide in G. lucidum and Poria cocos (Yin et al., 2021), are common macromolecular immunomodulatory substances in herbal drugs. Several studies have shown that plant polysaccharides have multi-channel and multi-level regulatory effects on the immune system. They can activate several immune cell types, including T cells, B cells, macrophages, and natural killer cells, activate complement, and promote the production of cytokines, thereby regulating many components of the immune system. For example, plant polysaccharides can activate macrophages by recognizing and binding specific receptors on the cell surface, thereby initiating the immune response (Yin et al., 2019). Fungal β-glucans from G. lucidum or certain edible fungi can indirectly resist cancer cells or tumors by activating immune cells. In addition, β-glucan alleviates allergy by reducing the pro-inflammatory cytokines, IL-6 and TNF-α, and increasing the production of antioxidants (Murphy et al., 2020), a mechanism of action that is similar to that induced by pattern recognition receptors.

Saponins are one of the secondary metabolite groups in medicinal herbs, exhibiting immunomodulatory properties using a variety of mechanism. Saponin compounds have been used to treat autoimmune diseases, allergic disorders and cancer. For example, Astragaloside IV can induce the differentiation of monocytes into M1 macrophages and initiate an anti-tumor immune response (Min et al., 2022). Glycyrrhetinic acid can alleviate allergic reactions in an asthma mouse model by inhibiting the production of the Th2 cytokines IL-5 and IL-13 (Kim et al., 2017). Ginsenoside Rh₂ can enhance the infiltration and cytotoxicity of T lymphocytes in tumors, improving the cellular immune function of tumor-bearing mice (Wang et al., 2017). Ginsenoside compound K (CK) can inhibit the abnormal activation of T lymphocytes in mice with collagenous arthritis, and suppress memory T cells, preventing the immune system from rejecting heart transplantations (Wang et al., 2009; Liu et al., 2014).

Polyphenols are plant-based organic compounds with a three-membered flavan ring structure. These substances are known for their health-promoting qualities, including scavenging free radicals, lowering oxidative stress, and reducing inflammation (Abbas et al., 2017). Phenolic compounds acting as antioxidants and anti-inflammatory agents help to mitigate oxidative stress, preserve the dynamic equilibrium of the intestinal inflammatory response and promote immunological tolerance (Williamson, 2017). Flavonoids and phenolic acids are the primary polyphenol family members. Flavonoids have a possible immunomodulatory effect on several illnesses, including autoimmune diseases and cancers (Ginwala et al., 2019). They have also been found to modulate key enzymatic functions of immune cells and mediate their immune-modulating activity (D'Arcy, 2022). Other natural phenolic compounds, such as resveratrol, also exhibit excellent immunostimulatory activities, increasing the infiltration of immune cells, regulating the phenotype, and promoting the secretion of related cytokines (Alesci et al., 2022).

Gamma-aminobutyric acid (GABA) is a kind of natural relaxant amino acid, that inhibits anxiety, depression, hypertension, and helps to regulate hormone secretion. GABA is found in the seeds, rhizomes, and tissue fluids of plants such as legumes, ginseng, and other herbal medicines. Dietary GABA improves non-specific immunity and reduces immune hyperactivity induced by lipopolysaccharide in adolescents (Zhang et al., 2022). Prior studies have shown that low physiological concentrations of GABA can inhibit T-cell growth by activating T-cell- specific GABA channels to suppress the immune response, a role that is promising in the treatment of autoimmune diseases (Bjurstöm et al., 2008; Prud'homme et al., 2015). Another study found that the oral administration of GABA can enhance the immune response under stress conditions (Abdou et al., 2006). Thus, appropriate supplementation with GABA may be beneficial to human health.

Probiotic bacteria secrete polysaccharides with a wide range of structures, compositions, and functions into the external environment. These are referred to as exopolysaccharides (EPS), most of which possess a broad spectrum of antitumor, immunomodulatory, glycemic control, and antioxidant effects (Nwodo et al., 2012). The immunoregulatory function of EPS strongly correlates with the chemical structure and configuration of these bacterial exopolysaccharides (Rajoka et al., 2020). More specifically, Lactobacillus EPS contain hydroxyl, phosphate, carbonyl, and other groups which promote their biological activities (Rajoka et al., 2020). In response to EPS from Lacticaseibacillus rhamnosus KL37, mouse peritoneal macrophages produce pro-inflammatory cytokines such as TNF-α, IL-6, and IL-12, and anti-inflammatory cytokines like IL-10, indicating that these compounds induce significant immunological activity (Ciszek-Lenda et al., 2011). Another study isolated and purified an acidic extracellular polysaccharide from Lp. Plantarum JLAU103, designated EPS103, with strong dual immunoregulatory activity (Wang et al., 2020). EPS103 was found to activate RAW264.7 macrophages while simultaneously reducing the excessive release and mRNA expression of several inflammatory factors, including IL-6, TNF-α, prostaglandin E2 (PGE2), NO, cyclooxygenase-2 (COX-2), and inducible NO synthase (iNOS) (Wang et al., 2020). These results suggest that LAB-specific EPS may act as immunomodulatory agents.

Anaerobic bacteria in the colon ferment indigestible dietary fiber into short-chain fatty acids (SCFAs) such as acetate, propionate, and butyrate. These, in turn, serve as energy sources and participate in a variety of signaling processes (Russell et al., 2013). A recent study found that microbiota-derived SCFAs play a critical role in inducing IL-22 production from CD4⁺ T cells and innate lymphoid cells (ILCs) by inhibiting G-protein receptor 41 (GPR41) and histone deacetylase (HDAC) to maintain intestinal homeostasis (Yang et al., 2020). Another study found that both lactate and SCFAs could downregulate proinflammatory responses in intestinal epithelial cells and sentinel cells, including myeloid cells, by inhibiting TLR signaling in a dose-dependent manner, thereby restoring gastrointestinal homeostasis (Iraporda et al., 2015). The SCFA, pentanoate, was shown to reduce T-cell mediated immunopathology in the intestine and brains of mice by promoting IL-10 production and suppressing Th17 cells. Thus, pentanoate may have potential in the treatment of inflammatory and autoimmune diseases (Luu et al., 2019).

As they proliferate, LAB creates antimicrobial proteins or polypeptides called bacteriocins, which have been used as both food preservatives and treatments for foodborne infections (Zacharof and Lovitt, 2012). The use of bacteriocins as possible therapeutic antibiotics or immunomodulators is gaining greater attention (Huang et al., 2021). Bacteriocins produced by probiotics can also have a positive effect on the immune system. In addition to reducing the host inflammatory response by directly antagonizing pathogens, some bacteriocins can activate immune cells and enhance immunity (Peters V. et al., 2019). Nisin, produced by Lactococcus lactis, is shown to enhance macrophage-induced IL-12 production (Moein et al., 2018), modify the levels of inflammatory factors in a bidirectional manner, and maintain the immunological balance (Małaczewska et al., 2019). Sublancin, an S-linked glycopeptide, is also found to enhance macrophage function and promote CD4⁺ and CD8⁺ cell proliferation (Wang et al., 2018; Wang J. et al., 2019). This glycopeptide can inhibit nuclear factor-κB (NF-κB) activation to relieve intestinal inflammation (Hu et al., 2018). G protein-coupled hydroxy-carboxylic acid receptors (HCARs) regulate immune function and energy balance when metabolism and dietary conditions change (Offermanns, 2017). D-3-phenyllactic acid (D-PLA), an antibacterial metabolite produced by LAB, specifically recognizes the human and ape-specific HCA3 receptor and transmits signals to immune cells to induce immune responses (Peters A. et al., 2019). However, whether HCA3 activation by D-PLA can affect monocyte phagocytosis or differentiation into macrophages requires further study.

Different natural products are thought to affect LAB growth in distinct ways. In general, polysaccharides can be used as a source of energy to facilitate bacterial growth, reproduction and metabolic processes, including the production of SCFAs. SCFAs are produced by glycolysis through specific enzymes from a range of bacteria (Macfarlane and Macfarlane, 2003). Their production is associated with multiple factors, including strain species, growth status, and substrate properties (Özcelik et al., 2016; Annunziata et al., 2020; Hadinia et al., 2022). Fermentation in vitro is recognized as a feasible strategy for obtaining large amounts of SCFAs. The SCFAs, acetic, propionic, and butyric acid were produced by four LAB strains isolated from Korean individuals and fermented foods, of which Bifidobacterium bifidum MG731 induced 4,998.6 μg·g⁻¹ SCFA production, Bifidobacterium lactis MG741 induced 2,613.9 μg·g⁻¹, Ligilactobacillus salivarius MG242 induced 1,456.1 μg·g⁻¹, and Lp. plantarum MG989 induced 630.2 μg·g⁻¹ (Kang et al., 2010). In addition, fructan and galactan from Polygonatum cyrtonema were shown to be selectively utilized by Bifidobacterium and Ligilactobacillus probiotic strains, highlighting their growth-promoting properties (Zhang J. et al., 2021). Polygonatum sibiricum polysaccharides primarily composed of rhamnose, arabinose, and xylose, may be degraded into monosaccharides and produce acetic acid, propionic acid, and lactic acid by intestinal microbial fermentation (Luo et al., 2022). Sarep, a valuable ingredient obtained from orchid tubers, was shown to exhibit prebiotic potential because of its glucomannan content, providing a carbon source for the growth of Bifidobacterium infantis and promoting the production of its SCFA metabolites (from 1.27 g·L⁻¹ to 1.94 g·L⁻¹) (Usta-Gorgun and Yilmaz-Ersan, 2020). G. lucidum polysaccharides significantly increased the relative abundance of Ligilactobacillus, Bifidobacterium, and SCFAs in the intestine of colitis mice, by activating G-protein-coupled receptor 43 (GPR43) in gut immune cells and eliciting anti-inflammatory and tumor immunoregulatory effects (Guo et al., 2021). A recent study found that polysaccharides from Polygonatum kingianum reinforced quorum sensing signaling in Ligilactobacillus faecis, and regulated the expression of genes and proteins related to the synthesis and metabolism of SCFAs, including upregulating the ldh gene, metE gene, and adh2 protein and downregulating the mvK gene to impact the production of intestinal SCFAs (Yang et al., 2021). Most studies of the impact of herbal ingredients on the metabolic activity of LAB are focused on the SCFAs. Meanwhile, the effect of other immunoactive ingredients, such as EPS and bacteriocins, in herbal medicines remains to be explored.

Macrophages are a type of innate immune cells that play a critical role in maintaining homeostasis and resisting foreign pathogens (Lee et al., 2020). These cells act as immune effectors that directly eliminate foreign pathogens and tumor cells and are also integral antigen presenting cells (APCs), that activate immune responses by secreting pro- and anti-inflammatory cytokines and chemokines (Aminin and Wang, 2021). Macrophage Fcγ receptor (FcγR) promote phagocytosis by aggregating where these cells come into contact with exogenous antigens. LAB fermentation can promote phagocytosis by activating FcγR and TLR2 signaling (Kwon et al., 2018), and inducing macrophages to produce TNF-α and IL-6, cytokines involved in innate immune regulation (Wang and He, 2020). Kim et al. (2019) reported that after Bifidobacterium animalis subsp. lactis-induced fermentation of red ginseng, macrophage activity was significantly enhanced by mitogen-activated protein kinase (MAPK) and NF-κB signaling. Red ginseng fermentation also activated central immune cells such as splenic cells and bone marrow-derived macrophages (BMDM) by promoting the production of TNF-α and IL-6. NO secreted by macrophages also induces immune cell activation and promotes resistance to pathogens (Hu et al., 2021). Mo et al. (2021) observed that Lacticaseibacillus paracasei HY7017 cultivated in 3% RGE-supplemented medium dramatically boosted NO production. In addition, the mRNA level of iNOS, a key enzyme in NO synthesis, and COX-2 were higher after treatment with HY7017-RGEs. This process had no effect on the secretion of the anti-inflammatory cytokine IL-10, indicating that fermented red ginseng enhanced the immune response without creating excessive inflammation (Mo et al., 2021).

Dendritic cells (DCs) are a type of professional APCs that function as immune sentinels and help to initiate immune responses (Zanna et al., 2021). Immature DCs possess a strong migration ability, and mature DCs can effectively activate naive T cells, involved in the initiation, modulation and maintenance of immune responses (Zhou and Ye, 2021). Mature DCs synthesize high levels of NO, IL12, and the chemokine, RANTES, and express co-stimulatory molecules, such as CD86 and major histocompatibility complex (MHC), to induce full T cell activation (Zhu et al., 2019). Several studies have shown that LAB-fermented herbal drugs regulate immunity by promoting DC maturation, likely through the activation of MAPK-JNK, p38 MAPK, and NF-κB signaling pathways (Zhang et al., 2009). In a mouse model, FAPS was shown to impact DC maturation-related signaling, thereby promoting their maturation (Bian, 2017). The findings revealed that different doses of FAPS (50, 100, and 200 μg·mL⁻¹) induced bone marrow-derived DCs to produce higher levels of IL-12, TNF-α, IL-1β, and IL-6 and related gene expression than unexposed DCs. By activating DCs, FAPS was also shown to promote 6.8–19% higher T cell proliferation (Bian, 2017). Another study found that DCs treated with fermented herbal medicines could directly stimulate B cell proliferation and differentiation. Zhang et al. (2009) reported that fermented Noni exudate (FNE) promoted the proliferation of mature DC and mixed spleen cell-cultured B lymphocytes, indicating that FNE can induce bone marrow-derived DC maturation and stimulate DC-induced immune responses required to promote B cell differentiation and immunoglobulin class conversion.

Natural killer (NK) cells, effector lymphocytes of the innate immune system, suppress tumors and microbial infections by patrolling for aberrant cells that lack MHC class I or overexpress NK cell receptors-specific ligands (Lanier, 2005; Vivier et al., 2008). After recognizing these cells, activated NK cells release granzymes and perforin to mediate target cell killing, and secrete several cytokines such as IFN-γ, TNF-α, and IL-10 that aid immunomodulation (Kim et al., 2007). Recent studies on autoimmune, transplantation and viral infections have linked NK cells to the control of T cell-mediated responses (Zwirner et al., 2021). LAB and its fermented products are shown to enhance NK cells activity and cytotoxicity. One study found that heat-killed Lactobacillus acidophilus La205 can directly stimulate NK cell cytotoxicity by increasing granule exocytosis (Cheon et al., 2011). Kang et al. (2010) assessed the effect of fermented ginseng extract on NK cell activity by measuring its cytotoxic impact on mouse lymphoma cells (Yac-1). The extract was shown to enhance NK cells cytotoxicity and promote IFN-γ secretion in a dose-dependent manner (Kang et al., 2010).

The complement system involves a cascade of enzymatic reactions composed of ~40 serum proteins. There are three complement activation pathways, classical, alternative, and lectin (Garred et al., 2021). The alternative pathway, also known as the bypass pathway, directly activates C3 by microorganisms or foreign bodies and is not dependent on antibodies. The complement components, factors B, D, and P, help to induce a cascade of enzyme-catalyzed complement reaction pathways that lead to the production of C3 invertase and C5 invertase, the formation of membrane attack complex (MAC) and the destruction of target cells (Gupta and Tripathy, 2020). Fragmented polysaccharide molecules produced by LAB-fermentation are shown to activate complement through the alternative pathway and regulate immune responses (Watanabe et al., 2015; Mastuba et al., 2017; Chen et al., 2022).

Humoral immunity refers to adaptive immune defense mechanisms that are mediated by antibodies produced by B lymphocytes. Antigen stimulation transforms B cells into antibody-producing B cell lines and plasma cells that produce antibodies, namely immunoglobulin (Ig), that specifically bind to the antigen. IgG, an essential component of the humoral immune response, is the predominant Ig class present in human serum. IgG binding to Fc receptors on the surface of macrophages, neutrophils, and NK cells induces antibody-dependent cell-mediated cytotoxicity (ADCC) (Dixon et al., 2021). IgA is the most abundant immunoglobulin found in humans and exists in two forms, serotype and secretory (sIgA). IgA is primarily involved in maintaining mucosal homeostasis, primarily by interacting with IgA Fc receptor I (FcRI) (Mkaddem et al., 2014). IgM, a pentamer containing a joining chain polypeptide, is the first antibody produced after infection and is often regarded as the first line of immune defense (Hiramoto et al., 2018; Keyt et al., 2020). Red ginseng fermentation is shown to increase IgG and IgA levels in healthy human serum, potentially due to the presence of immunoreactive components such as polysaccharides and saponins (Kim et al., 2011). One study found that Lp. Plantarum fermentation of Gynostemma pentaphyllum oral liquid promoted the secretion of IgA, IgG, IgM and related cytokines into the serum of immunosuppressed mice and enhance their immunity (Du, 2021). These findings illustrate that LAB-fermented herbal medicines can enhance antigen- specific immune responses.

Cell-mediated immunity is an immune response that is independent of antibodies but dependent on the ability of antigen-specific T cells to recognize and eradicate intracellular pathogens. Upon stimulation by their homologous antigen, immature T cells proliferate and differentiate into effector cells, and then undergo apoptosis or survive as precursors of long-lived memory cells. IL-2 is a potent T-cell growth factor that stimulates T-cell growth, proliferation, and subsequent differentiation and enhances their cytotoxic effects (Malek, 2003; Rochman et al., 2009). Activated T cells express TNFSF14, a member of the TNF superfamily that plays a critical role in the communication system that regulates T cell responsiveness (Wang and Fu, 2004). Kim et al. (2015) studied the effect of mixed fermentation broth on T cell activation by fermenting ginseng buds with Lp. plantarum and Lacticaseibacillus casei. This broth was shown to inhibit inflammation by preventing the expression of IL-1β, and enhancing the expression of IL-2 and TNFSF14 in T cells (Kim et al., 2015). There are two types of CD4⁺ T cells, T helper type 1 (Th1) and regulatory T cells (Treg). Th1 cells produce IFN-γ and IL-2 and induce cell-mediated immunity (Murakami et al., 2004), while Treg cells are essential for maintaining self-tolerance and immune cell homeostasis (Göschl et al., 2019). RG fermentation products can promote the differentiation of splenic lymphoid T cells into Th1 and Treg cells, reducing immunosuppression (Kim et al., 2018). The CD4⁺ / CD8⁺ ratio is a key indicator of immune function. Lp. plantarum NCU137-fermentation of Coix seed was found to significantly increase the splenic index, restore the CD4⁺/CD8⁺ ratio of splenic T lymphocyte subsets, down-regulate TNF- α and IL-6 production, and reduce inflammation, and the effect was better than that of unfermented group (Wang et al. 2022). Mixed probiotics (Lactobacillus acidophilus and Bifidobacterium breve)-mediated fermentation of G. lucidum fruiting bodies increased the proportion of CD4⁺ T and CD8⁺ T lymphocytes in mouse spleen and improved T cell transformation (Li et al., 2021).