Mannan may refer to a plant polysaccharide that is a linear polymer of the sugar mannose. Plant mannans have β(1-4) linkages. It is a form of storage polysaccharide. Ivory nut is a source of mannan.
Mannan may also refer to a cell wall polysaccharide found in yeasts. This type of mannan has a α(1-6) linked backbone and α(1-2) and α(1-3) linked branches. It is serologically similar to structures found on mammalian glycoproteins. Detection of mannan leads to lysis in the mannan-binding lectin pathway. This mannan is the source of MOS used as prebiotics in animal husbandry and nutritional supplements.