Beta-glucans and the dectin-1 and TLR receptors
Reishi (Ganoderma lucidum) is a medicinal mushroom whose immune-related effects are usually attributed to its polysaccharides, particularly beta-glucans. These long-chain sugars are recognized by pattern-recognition receptors on innate immune cells, most notably dectin-1 and certain Toll-like receptors (TLRs). Binding these receptors is a normal way the body detects fungal cell-wall material, which is why beta-glucans engage the immune system rather than being inert fiber.
Training innate immunity versus stimulating it
The proposed effect is sometimes described as immune training: priming innate cells such as macrophages and natural killer cells so they respond more readily, rather than producing a single sharp activation. Reishi also contains triterpenes (ganoderic acids) studied separately for other effects, so a reishi extract is a mixture, not a single compound.
Most immune findings come from cell, animal, and small human studies, so they are preliminary and do not establish that reishi prevents or treats infection or cancer. This is general educational information, not medical advice. Because immune-active compounds can theoretically interact with immunosuppressant drugs or autoimmune conditions, consult a qualified professional before use in those situations.