As the largest naturally occurring polymer in nature, polysaccharides exhibited a wide variety of structural and functional forms (1, 2). Advances and easy access to certain analytical tools help in the identification and quantification of polysaccharides, thereby supporting in-depth studies to determine the biological activities of polysaccharides. The polysaccharides shared certain structural features, just like other polymers such as proteins and nucleic acids (DNA, RNA), in which monosaccharide residues were linked together by glycosidic bonds (3). There are major differences in the structure of polysaccharides, which perform numerous biological functions at the cellular and tissue levels. In oligosaccharides and polysaccharides, the monosaccharide molecules can reassemble multiple times to form countless linear or branched structures, each with different functions. Polysaccharides, like proteins, exhibit the highest structural diversity ever known, with cosmopolitan distribution in nature, where they perform a variety of functions, as building blocks in cell membranes, energy storage, cell recognition, differentiation, proliferation and control of cell signaling (4–7). The biological properties of polysaccharides have attracted increasing attention in biochemistry and medicine in recent decades.

Polysaccharides exhibit a wide range of biological activities, including antioxidant, anti-inflammatory, antidiabetics, gut microbiota regulation and immunomodulatory effects (8, 9). Polysaccharides have emerged as significant players in the field of immunology due to enhancing the innate and adaptive immune response of the host. Polysaccharides stimulate immune cells via specific receptors, for example in necrophage and macrophage cells via bonds between functional groups of polysaccharides and groups of molecules on the cell surface. As soon as the polysaccharides bind to membrane receptors in the defense cells, the signaling pathways are activated and a cycle of biochemical processes begins, which leads to a positive regulation of gene expression in the ribosomes and initiates protein production. Moreover, polysaccharides influence immunity by modulating cytokines that have pro- and anti-inflammatory effects. They activate macrophages via signaling pathways such as TLRs and NF-κB and thus improve immunological functions. The ability of polysaccharides to enhance macrophage function is a crucial aspect of their immunomodulatory potential, as these cells are pivotal in the body’s defense mechanisms. Certain polysaccharides, such as those from Tinospora cordifolia, have been shown to increase macrophage activity, while Bupleurum polysaccharides have been shown to reduce inflammation (10). T lymphocytes produce cytokines that influence inflammatory responses, such as IL-10 and IL-4 (Th2) or TNF-α and IFN-γ (Th1). The mycelium of PhomaherbarumYS4108 contains polysaccharides that stimulate T cells via TLR2/4, improving immunological function. Polysaccharides from Ganoderma lucidum activate B cells, resulting in an increase in immunoglobulin synthesis. Neuroinflammation is reduced by fucoidan treatment, blocking the proinflammatory response of microglia (11). Additionally, the interaction between polysaccharides and the gut microbiota has garnered attention for its implications in immune health. Gut microbiota produces huge number of metabolites, consequently by metabolizing and anaerobic fermentation of complex polysaccharides. Short-chain fatty acids (SCFAs) may help maintaining immunological homeostasis and immune cell functions both locally and systemically. Polysaccharides improve antioxidant capacity, reduce the production of pro-inflammatory mediators, strengthen the intestinal barrier, influence the composition of intestinal microbial populations, and promote the synthesis of SCFAs (12). The potential of polysaccharides not only as direct immunomodulators but also as agents that can influence the broader immune landscape through gut health.

This review highlights the structural features, immunomodulatory properties, underlying immunomodulatory mechanisms of naturally occurring polysaccharides, and activities related to immune effects by elucidating a complex relationship between polysaccharides and immunity.

Polysaccharides are complex carbohydrates that play crucial roles in various biological processes across different organisms, including plants, animals, and microorganisms. Their structural differences significantly influence their biological activities, which can range from antioxidant properties to immunomodulatory effects. The molecular weight, number of branched chains, percentage of monosaccharides, and structural conformations of each polysaccharide source vary. As shown in Figure 1 , types and structural arrangement of the common natural polysaccharides. Cellulose, starch, inulin and pectin are abundant in plant tissue. Animal polysaccharides include glycogen, chitosan and chitin. Glucan polysaccharides are widespread in mushrooms. Microbial polysaccharides include pullulan, xanthan gum, dextran, lentinan and curdlan. Exopolysaccharides are polysaccharides released by microorganisms. Polysaccharides from natural sources with structural diversity and different biological activities ( Table 1 ). The structural differences among plant, animal, and microbial polysaccharides lead to distinct biological activities. Understanding these relationships is crucial for the development of polysaccharide-based therapeutics and functional foods, as their diverse structures can be tailored to enhance specific biological functions.

The influence of natural polysaccharides on the immune system has received more attention. The ability of polysaccharides to alter immunity depends primarily on the presence of unique structures such as branching regions, molecular weight, acetyl or sulfate groups, conformation, monosaccharide composition, and glycosidic linkages (56). The molecular weight of polysaccharides plays a crucial role, while lower molecular weight polysaccharides often show stronger immunomodulatory effects. Each polysaccharide has a significant molecular weight, and different compounds with different molecular weights affect immune regulation in different ways. Research found that the ability of Schisandrachinensis polysaccharides to alter the immune system is inversely correlated with their molecular weight (57). The reduced molecular weight and simpler structures of polysaccharides provide an advantage over other types of molecules and can easily pass through cell barriers (58). The effect of different polysaccharide molecular weights on immunity is complex and important. Immune stimulation: Low molecular weight polysaccharides typically have stronger immune stimulating effects. They have a greater ability to activate immune cells, including natural killer cells, dendritic cells and macrophages, which increases the production of cytokines and other signaling molecules involved in immunological responses (59). Pattern recognition: Low molecular weight polysaccharides often have simple structures that may be found more easily by the pattern recognition receptors (PRRs) of immune cells. This identification triggers innate immune responses and allows the adaptive immune system to mount a stronger defense against infection (60). Phagocytosis Activation: Low polysaccharides can improve the ability of immune cells to absorb and clear infections through a process called phagocytosis (61). This increases the overall effectiveness of the immune response and helps remove pathogenic organisms. Antiviral and antitumor activity: Some low molecular weight polysaccharides have been demonstrated to have potent antiviral and anticancer properties (62, 63). They can both directly stop the growth of tumor cells and viruses and strengthen the immune system for an intensive attack on diseases or cancer cells. Immunomodulation: Polysaccharides of different molecular weights can regulate immunological reactions in different ways. Smaller polysaccharides can have immunosuppressive effects, but larger polysaccharides typically promote immunological activation (63, 64). In diseases such as autoimmune diseases, which are characterized by dysregulated immune responses, this regulation can be used therapeutically.

In addition, branched polysaccharides tend to have higher immunomodulatory activity compared to linear ones. The highly branched side chains of pectin may influence the immunological regulatory effects of pectin on LPS-induced IL-6 production, as shown in in vitro studies (65). Research has shown that pectin can have an even stronger inhibitory effect on LPS-induced cytokine production when its rhamnogalacturonan I side chains are removed (66). At the same time, it was found that pectin polysaccharides with sulfate groups stimulated a number of neutrophil and macrophage effector functions (67). Standard polysaccharide moieties are structurally linked to cores that have linked chains or linked monosaccharide residues (56). Different functional groups and patterns trigger different immune responses, but sometimes high levels of acetylation result in impaired immune activity (68). Depending on their solution conformation, polysaccharides can interact with immune system cells or other components in different ways. These interactions can be single, triple or random coils. For example, the researchers found that the triple helix conformation promoted the production of TNF-α by immune cells, which enhanced the immunomodulatory effect on β-glucans (69).

The composition of monosaccharides in polysaccharides plays a crucial role in determining their biological activities, particularly their immunomodulatory effects. The immunoenhancing activity of a novel heteropolysaccharide isolated from custard apple pulp was closely related to its monosaccharide composition, particularly noting that specific monosaccharides like glucose and rhamnose can enhance immune responses in mouse macrophages and dendritic cells (70). Moreover, the specific types of monosaccharides present in polysaccharides can dictate their interactions with immune cells. The polysaccharides from Ganoderma atrum could induce anti-tumor activity through the activation of host immune responses, with the composition of monosaccharides such as mannose and galactose being pivotal in mediating these effects (71). Another study result further supports this notion, indicating that polysaccharides containing domains of specific monosaccharides like galactose and mannose exhibit significant immunostimulatory activity (72). Moreover, wang reviewed the effects of monosaccharide composition and proportion on the bioactivity of polysaccharides, emphasizing that variations in these parameters can lead to significant differences in their immunomodulatory effects (73). the monosaccharide composition of polysaccharides is a fundamental determinant of their immune activity. Variations in the types and proportions of monosaccharides not only influence the structural characteristics of polysaccharides but also their interactions with immune cells, leading to diverse biological effects.

Glycosidic linkages are fundamental to the structure and function of polysaccharides, which play crucial roles in various biological processes, including immune responses. The type and configuration of glycosidic linkages significantly influence the immunogenicity and biological activity of polysaccharides. The polysaccharides with α-1,6-linkages have been shown to enhance immune responses, as seen in studies involving α-glucans derived from microbial sources, which are essential for macrophage phagocytosis and the induction of innate immune responses through Toll-like receptor 2 (TLR2) pathways (74). The structural diversity of glycosidic linkages also contributes to the functional variability of polysaccharides. For example, the presence of different glycosidic bonds, such as β-1,3 and β-1,4 linkages in galacto-oligosaccharides, has been associated with various biological activities, including prebiotic effects and modulation of immune responses (75). Furthermore, polysaccharides derived from plants, such as those from Radix Aconiti, have demonstrated immunostimulatory effects, enhancing the host’s immune response and showing potential as biological response modifiers (74). Glycosidic linkages are pivotal in determining the immune activity of polysaccharides. The type and configuration of these linkages influence the immunogenicity of polysaccharides, their ability to activate immune responses, and their potential applications in vaccine development and therapeutic interventions. Scientists have found a variety of potential sites for the production of immune-regulating polysaccharides that could be used in functional foods. Hopefully, further studies on the structure-function relationships of immunomodulation including polysaccharides will be carried out soon. Overall, the structural diversity of polysaccharides plays a significant role in modulating immunoregulatory functions (76).

Stimulating the complement system of dendritic cells and macrophages is the primary mechanism by which polysaccharides influence immunity ( Figure 2 ). The increase in the number of lymphocytes and the immunological organ indices is within the scope of their possibilities. There are a number of signaling pathways and receptors on immune cells that also activate them (77). In addition to stimulating cytokines and complements, polysaccharides also stimulate the production of immune cells including macrophages, T cells, B lymphocytes, and (NK) cells, among others (78). Macrophages play a crucial role in the defense mechanisms of the host immune system (79, 80). In particular, polysaccharides influence the immunological responses of macrophages by increasing phagocytic activity, cell proliferation, ROS production and cytokine secretion (81).

Intercellular adhesions, cell division and cell proliferation are all regulated by cytokines. Interleukin (IL), colony stimulating factor (CSF), tumor necrosis factor (TNF), and interferons (IFN) are the four classes of cytokines. In addition to their essential functions in regulating inflammatory and immunological responses, polysaccharides also regulate adaptive and innate immunity (80). The polysaccharide component CPE-II can significantly increase the production of the pro-inflammatory cytokines TNF-α and IL-6 as well as the anti-inflammatory cytokine IL-12 in the macrophage cell line RAW 264.7 (82). Polysaccharides can simultaneously control the production of pro-inflammatory and anti-inflammatory cytokines. The fact that IL-12 acts as a negative feedback mechanism during the hyperinflammatory response to prevent overactivation of macrophages lends credence to the idea that the body has a self-regulatory system that keeps everything in balance. Likewise, cytokine production by macrophages can be induced by polysaccharides (83, 84).

Natural polysaccharides and cytokines can combine to stimulate the immune response of macrophages. Macrophages can release many cytokines such as NO, IL-1β and TNF-α when the polysaccharides SHP and IFN-γ work together. The transcript levels of the cytokines IL-1 and TNF-α also increased significantly. The synergistic effect also altered the differentiation markers CD11b, CD18 and CD24 produced by macrophages (85). Complement, through its numerous common activation pathways, plays an essential role in immunological control and the body’s response to microbial defense. Immunological effects, control of the complement system, and complement modulators for complement-related diseases are all aspects of sulfated glycosaminoglycans (86, 87). Fruits of the species Capparis spinosa L. contain sulfated polysaccharides that can inhibit both traditional and nontraditional pathways of complement activation, making them a promising candidate for therapeutic complement suppression (88). These processes include the many complement pathways, the mannose-binding lectin (MBL) pathway, and classical complement (89). Ca⁺² is essential for the MBL protein. By binding to the mannose receptors of certain pathogen cells, lectins activate the MBL complement system, which in turn generates immunity (90).

Biological response modifiers (BRMs) are polysaccharides that possess immunomodulatory properties ( Figure 3 ), allowing them to simultaneously influence innate and adaptive immune responses (91). Molecular recognition of polysaccharide BRM is achieved by proteins such as plasma proteins and pattern recognition receptors (PRRs). Pathogen-associated molecular patterns (PAMPs) are PRRs – nonclonal immune proteins – that identify common molecular features among different microbes. Beginning with their binding to PRRs, ligands activate genes of innate immunity through signaling cascades regulated by Rel and NF-κB (92).

Two different types of PRRs can be used for polysaccharide BRMs: scavenger receptors (SRs) and toll-like receptors (TLRs) (93). In order for TLRs to interact with intracellular signaling proteins and accept ligands, they have two unique features: the Toll/IL-1 receptor-like domain and leucine-rich repeats (94). Recognizing both endogenous ligands that support tissue homeostasis and immune defense signaling is the role of multidomain and multiligand receptors (SRs). Both macrophages and dendritic cells (DCs) produce SRs (94). The BRMs are identified by class A SR, β-glucan receptors, mannose receptors and complement receptor type 3 (CR3). Fucoidan is a polysaccharide ligand related to class A SR (95). A recognized binding site for Dectin-1,3-Glucan is the glucan receptor/Dectin-1 (96). The mannose receptor requires calcium for binding of mannosyl/fucosyl or GlcNAc glycoconjugate ligands (97). Both the CR3 membrane receptor and the adhesion molecule are essential. As previously mentioned, β-glucan, fixed iC3b and intercellular adhesion molecule-1 (ICAM-1) are the ligands that CR3 recognizes as a receptor (98).

The MBL of the lectin-complement pathway and proteins from the alternative and conventional complement pathway are among the proteins in blood plasma that can recognize polysaccharide BRMs. As a multimericlectin, mannose-binding lectin can detect multiple diseases without the need for a specific antibody (99). Activation of the lectin complement cascade occurs upon MBL attachment to macromolecules containing fucose, mannose, N-acetylglucosamine, and glucose residues that are aligned and densely packed like microorganisms. As a result, the body begins to respond by releasing inflammatory chemicals (100).

Polysaccharide BRMs can trigger specific cellular and molecular events and produce specific expression patterns depending on the receptors and binding proteins to which they bind. The receptors and binding proteins present in polysaccharide BRMs, including mannose receptors, CR3, β-glucan receptors, TLRs, SRs, lectin-binding pathway binding proteins, and the alternative and classical complement pathways, originate from innate immunity, as previously mentioned. Not only mast cells, neutrophils and epithelial cells express TLRs, but also myeloid cells (monocytes, macrophages and dendritic cells) (101). Two Toll-like receptors, namely TLR-2 and TLR-4, are known to recognize polysaccharide BRMs, out of ten Toll-like receptors found in humans (102).

Endothelial cells, DCs, smooth muscle cells, and macrophages all express SRs (103). To SR’s knowledge, fucoidan is one of the polysaccharide BRMs. Various cell types, such as B and T lymphocytes, neutrophils, DC, eosinophils and monocytes, express the β-glucan receptor (95, 104). Certain cell types, including DCs, hepatic endothelial cells, renal mesangial cells, macrophages, and tracheal smooth muscle, express mannose receptors (105). The mannose receptor is bound by the mannan-type polysaccharide BRM. Cells that primarily express CR3 are macrophages and neutrophils. Furthermore, CR3 is expressed by a tiny percentage of B and T cells as well as non-NK cells (106). Myeloid cells are the main targets of polysaccharide BRMs as they are essential for innate and adaptive immunity (107). One of the most important types of myeloid cells, macrophages, are stimulated by BRMs to increase their phagocytosis, ROS production, cytokine release and activation marker expression (such as FcR and B-7) (108, 109). As a result, polysaccharide BRMs improve effector function, antigen processing ability and regulation of acquired immunity by promoting cytokinesis, antigen presentation and expression of macrophage cell adhesion molecules. Therefore, polysaccharide BRMs can secondarily or consequently directly stimulate other immune cells such as lymphocytes and NK cells. In many cases, BRMs containing polysaccharides are very potent mitogens (80, 110).

Activation of the polysaccharide BRM leads to an increase in macrophages, NK cells, T lymphocytes and B lymphocytes. This happens, among other things, through the binding of polysaccharides to their receptors. Mitogen-activated protein kinases (MAPKs), involved in intracellular signaling events, are responsible for mitogen stimulation of macrophages ( Figure 3 ), B lymphocytes, and NK cells by polysaccharide BRMs (111). Through their immunomodulatory function, BRMs promote the proliferation of immune cells and inhibit cell death (112). BRMs not only activate cells, they also stimulate the immune system’s complement pathways. One way the active complement system helps clear infections is by activating macrophages (113). Members of the β-glucan family comprise the polysaccharides with the greatest biological activity. Since they are PAMPs, glucans can be identified by various cell surface receptors on monocytes/macrophages, neutrophils, (NK) cells, DCs, T cells and B cells (114, 115).

In the presence of polysaccharides, interaction with CR3 leads to upregulation of NF-κB, activation of PI3K, and phosphorylation of Syk, the spleen tyrosine kinase. As a result of these events, cytokines such as interleukin IL-2, IL-10 and TNF-α are released (116). Various cells such as monocytes, macrophages, neutrophils, T cells and dendritic cells are known to express Dectin-1. β-Glucans promote innate immune responses (phagocytosis, reactive oxygen species production, IL-12, IL-6, IL-10 and TNF-α synthesis) by activating many signaling pathways such as Syk, Akt, MAPK and nuclear factor activated T cells (NFAT) (116, 117). Another type of receptor that binds polysaccharides and can identify β-1,3-glucans is lactosylceramide or (LacCer). LacCer expression is abundant in neutrophils, dendritic cells and macrophages. Together, polysaccharide and LacCer enhance the oxidative burst response of neutrophils, stimulate macrophage inflammatory protein-2, and activate NF-κB (118). Dendritic cells and macrophages have scavenger receptors that can identify fungal β-glucans. However, the exact mechanism by which these receptors are triggered is still unknown. It is generally believed that activation of the MAPK and PI3K/Akt kinases occurs through ligand interaction (119, 120).

The immune activity of natural polysaccharides is closely related to intestinal immunity, anticancer, antigen recognition and presentation, antimicrobial and anti-inflammatory effects. As shown in Table 2 , other effects of polysaccharides related to immunity. It not only helps maintain immune homeostasis and the body’s defense ability, but also provides a broad perspective for polysaccharides in the field of drug development and clinical application. The immune activity of natural polysaccharides allows the body to build a solid defense net.

In conclusion, polysaccharides are vital biomacromolecules involved in many important biological activities. They have attracted great scientific interest due to their health benefits, including immune stimulation and immunomodulation, associated intestinal immunity, anticancer, antigenic, antibacterial and anti-inflammatory activities. The wide variety of polysaccharides found in nature is due to the many different species of plants, animals and microbes. Polysaccharides provide a rich source of bioactive compounds with significant potential for health and medicine. Their non-toxic nature and significant influence on biological functions, especially on immunological cells, make them valuable for the treatment and prevention of diseases. Polysaccharides hold promise as modulators of the host’s defense and immune systems, and their immunoregulatory functions are highly dependent on their structural features. Polysaccharide structure, molecular weight, acetyl or sulfate groups, and branching areas greatly influence how they interact with immune cells and other components of the immune system. In addition, polysaccharides are essential for gut-associated immunity as they modulate inflammatory responses, improve the integrity of the intestinal barrier, influence the composition of the gut microbiota, and promote the formation of SCFAs. Due to their ability to alter immune responses, induce apoptosis, and reduce tumor cell proliferation, polysaccharides have shown promise as potential anticancer drugs in cancer immunotherapy. Polysaccharides also have antimicrobial properties. They prevent bacteria from multiplying by preventing them from forming biofilms, forming cell walls and blocking their intracellular metabolic processes. By regulating the secretion of anti-inflammatory cytokines, limiting the expression of inflammatory mediators, and modifying the release of pro-inflammatory cytokines, they can also exert anti-inflammatory effects. Overall, polysaccharides represent a broad class of chemicals that have important implications for immunological function, host-microbiota interactions, cancer immunotherapy, antimicrobial defense, and inflammatory modulation. Further exploration of the structure-function relationships of polysaccharides and their medical applications promises to improve our understanding of immunology and develop new approaches to treating immune-related diseases.