Edible and medicinal mushrooms provide structurally distinctive polysaccharides, dominated by β-glucans with β(1 → 3) backbones and β(1 → 6) branches, whose molecular weight, branching, conformation, and solubility regulate bioactivity and vary with species and extraction, motivating rigorous chemotyping across studies. These polymers engage conserved pattern-recognition circuitry (notably dectin-1 → spleen tyrosine kinase (Syk) → CARD9–BCL10–MALT1(CBM) → nuclear factor kappa B (NF-κB), with complement receptor 3 (CR3)/toll-like receptor 2 (TLR2) crosstalk) and, beyond acute signaling, can imprint trained immunity; ex vivo work with Agaricus bisporus β-glucans shows canonical chromatin and metabolic reprogramming of human monocytes. Across indications, the most mature oncology data concern Trametes versicolor products (polysaccharopeptides (PSP) and polysaccharide-K (PSK)) and lentinan from Lentinula edodes: meta-analyses and cohorts in resected or advanced gastric cancer report survival benefits when lentinan is added to chemotherapy, and historical randomized trials with schizophyllan (sizofiran) plus radiotherapy improved tumor control in Stage II cervical cancer. In infectious-disease contexts, parenteral lentinan is used as an immunostimulatory adjuvant in parts of Asia, while oral pleuran (Pleurotus ostreatus) is positioned for respiratory support; recent multicenter pediatric data indicate improved asthma control and fewer respiratory infections, whereas a pediatric gastroenteritis randomized controlled trial (RCT) was negative, emphasizing indication-specific effects and disease-formulation matching. At the gut interface, mechanistic colitis models delineate epithelial (dectin–1–tumor necrosis factor receptor 1 (TNFR1)) and myeloid checkpoints targeted by oral lentinan. Convergent evidence indicates that β-glucans act as prebiotic-like substrates, increasing short-chain fatty acid (SCFA) production, reinforcing tight junctions, and attenuating endotoxemia. Neurocognitive signals include randomized trials of Hericium erinaceus in mild cognitive impairment (MCI)/early Alzheimer’s disease (AD) alongside cohort and biomarker data that implicate neurotrophic and microbiota–SCFAs pathways. However, key translational gaps persist in product heterogeneity, small underpowered trials, and inconsistent endpoints. This chapter therefore prioritizes (i) chemistry-anchored standardization, (ii) matrix-aware delivery (including colon-targeted strategies), and (iii) biomarker-linked clinical designs (trained immunity, barrier, and microbiome readouts) to enable dose–response modeling and responder stratification across oncology, infection, metabolism, gut, and brain health.