Fve is a major fruiting body protein from Flammulina velutipes, a mushroom possessing immunomodulatory activity. It stimulates lymphocyte mitogenesis, suppresses systemic anaphylaxis reactions and oedema, enhances transcription of IL-2, IFN-gamma and TNF-alpha, and haemagglutinates red blood cells. It appears to be a lectin with specificity for complex cell-surface carbohydrates. Fve adopts a tertiary structure consisting of an immunoglobulin-like beta-sandwich, with seven strands arranged in two beta sheets, in a Greek-key topology. It forms a non-covalently linked homodimer containing no Cys, His or Met residues; dimerisation occurs by 3-D domain swapping of the N-terminal helices and is stabilized predominantly by hydrophobic interactions.