CHAPTER 6 IMMUNOMODULATORY ACTIVITIES OF MUSHROOM GLUCANS AND POLYSACCHARIDE-PROTEIN COMPLEXES IN ANIMALS AND HUMANS Synopsis: This chapter focuses on the ability of extracts from medicinal mushrooms to stimulate or modulate the host immune responses. Numerous bioactive polysaccharides or polysaccharide-protein complexes from medicinal mushrooms are described that appear to enhance innate and cell-mediated immune responses, and exhibit antitumour activities in animals and humans. Stimulation of the host immune defence systems by bioactive polymers from medicinal mushrooms has significant effects on the maturation, differentiation and proliferation of many kinds of immune cells in the host. Many of these mushroom polymers were reported previously to have immunotherapeutic properties by facilitating growth inhibition and destruction of tumour cells. Recent research has also shown that some of these mushroom-derived polymers may possess direct cytotoxic effects on cancer cells. Whilst the mechanism of their antitumour actions is still not completely understood, stimulation and modulation of key host immune responses by these mushroom polymers appears central. Although somewhat controversial, recent evidence suggests that mushroom polymers (β-glucans) may trigger the stimulation of many kinds of immune cells in animals and humans by binding to a specific cellular receptor known as complement receptor type 3 or CR3.