Polysaccharides

Medicinal mushrooms and a handful of immune herbs account for most of the literature:

The answer is that the gut is an immune organ. Beta-glucans with a 1,3-linked backbone and 1,6 branches are recognized by pattern-recognition receptors — Dectin-1, complement receptor 3, TLR2 — on the immune cells of the gut-associated lymphoid tissue, particularly at the Peyer's patches. The plant molecule looks, to those receptors, like a fungal cell wall.

Binding there triggers a signalling cascade that propagates systemically without the molecule itself going anywhere. Activity depends heavily on linkage pattern, branching frequency and molecular weight — which is why "contains polysaccharides" on a label tells you essentially nothing, and why extraction method matters more here than for almost any other class.

The strongest clinical evidence is from Japan and China, where the mushroom polysaccharides lentinan and PSK have been used for decades as adjuncts to conventional cancer chemotherapy, with meta-analyses showing modest survival benefits in gastric and colorectal cancer. These are injected or given as standardized preparations, not as culinary mushrooms.

Beyond that: acemannan has a wound-healing and mucosal-protective literature, arabinogalactans act as prebiotics and modestly modulate immune markers, and the immune-herb polysaccharide fractions of echinacea and astragalus carry a good part of those herbs' attributed activity.