Immune system and immunomodulatory activity of β-glucans

The immune system operates seeking to protect the organism from infections that can be caused by various agents, including bacteria, viruses, fungi, and parasites. The cells and molecules of the immune system are highly specialized in the defense against infection. 

Individuals with a compromised immune defense system due to various factors, such as age, chronic infection, or malnutrition, are subject to several problems, including arthritis, reduced healing capacity, reduced proliferation of bone marrow cells with consequent low defense cell counts, anemia, and increased incidence of all types of microbial infections. Studies also show that one of the main elements of the process of aging is a decrease in the functional effectiveness of the immune system . Among the immunologically competent cells, macrophages play a major role in the initiation and maintenance of immune response both innate and adaptive . In addition to the functions of phagocytosis and the release of lysosomal enzymes, macrophages are also responsible for the release of a number of cytokines and inflammatory mediators can stimulate the immune system in general .

The β-glucan belongs to the class of substances BRMs, or a variety of different substances known as Biological Response Modifiers. being able to trigger a series of events in the immune response , increasing the immune defense of the host by activating the functions of cells of the immune system . This polymer is currently considered as one of the most potent stimulators of the immune response, effective both orally or intravenously, completely non-toxic and safe . The response of β-glucan in vertebrates begins with its recognition by receptors present on the cell surface of various immune cells, such as macrophages, neutrophils, dendritic cells, and natural killer cells (NK), and receptors have also been described presently as non-immune cells, endothelial cells, fibroblasts, alveolar epithelial, and Langerhans cells . The various receptors present on the cell membranes of immune cells related to the recognition of β-glucan in vertebrates are dectin-1, complement receptor 3 (CR3), lactosylceramide receptor, Toll-like receptor 2 (TLR-2), and scavenger receptors . 

The dectin-1 is a type II transmembrane protein with receptor extracellular domain CDR which is responsible for carbohydrate recognition, and a cytoplasmic tail with immunoreceptor ITAM (tyrosine-based activating motif) involved in superoxide production by macrophages in response to the immune system defense. The dectin-1 can mediate diverse cellular responses, including phagocytosis and endocytosis. This protein may also induce the production of cytokines and inflammatory chemokines, such as tumor necrosis factor (TNF-α), macrophage inflammatory protein-2 (MIP-2), and interleukin-12 (IL-12) . The receptor CR3 stimulates cytokine secretion by NK cells, especially in the presence of pathogens. This receptor acts as a cell adhesion molecule since it has a binding site for carbohydrates located on the terminal carbon, and thus a receptor for the phagocyte β-glucan .