Beta-glucans are long chains of sugar molecules that form part of the cell walls of mushrooms. Mushroom beta-glucans are counted as dietary fiber, and they are among the most studied compounds in medicinal mushrooms (Łysakowska et al., 2023; Wasser, 2002).
How the Immune System Sees Them
Immune cells recognize fungal beta-glucans through receptors on their surface. The best known is Dectin-1, found on macrophages and other immune cells (Brown & Gordon, 2003). In mice bred without Dectin-1, immune cells could no longer recognize beta-glucans properly and the animals were less able to control fungal infection (Taylor et al., 2007). This is how the body detects fungi; whether eating mushroom beta-glucans meaningfully changes immune function in people is a separate question, still being studied.
Getting Them Out of the Mushroom
Mushroom polysaccharides, including beta-glucans, are traditionally drawn out with hot water, which is why teas and hot water extracts carry them (Zheng et al., 2020). See Dual Extraction.
References
Brown, G. D., & Gordon, S. (2003). Fungal β-glucans and mammalian immunity. Immunity, 19(3), 311–315. https://doi.org/10.1016/S1074-7613(03)00233-4
Łysakowska, P., Sobota, A., & Wirkijowska, A. (2023). Medicinal mushrooms: Their bioactive components, nutritional value and application in functional food production. A review. Molecules, 28(14), Article 5393. https://doi.org/10.3390/molecules28145393
Taylor, P. R., Tsoni, S. V., Willment, J. A., Dennehy, K. M., Rosas, M., Findon, H., Haynes, K., Steele, C., Botto, M., Gordon, S., & Brown, G. D. (2007). Dectin-1 is required for β-glucan recognition and control of fungal infection. Nature Immunology, 8(1), 31–38. https://doi.org/10.1038/ni1408
Wasser, S. P. (2002). Medicinal mushrooms as a source of antitumor and immunomodulating polysaccharides. Applied Microbiology and Biotechnology, 60(3), 258–274. https://doi.org/10.1007/s00253-002-1076-7
Zheng, S., Zhang, W., & Liu, S. (2020). Optimization of ultrasonic-assisted extraction of polysaccharides and triterpenoids from the medicinal mushroom Ganoderma lucidum and evaluation of their in vitro antioxidant capacities. PLoS ONE, 15(12), Article e0244749. https://doi.org/10.1371/journal.pone.0244749