high molecular weight polysaccharides synthesised by some micro organisms. Consist of D glucose linked by _ 1,6 bonds (and a few _ 1,3 and _ 1,4 bonds). Dextran 75 (average molecular weight 75 kD) has a colloid osmotic pressure similar to blood Plasma, so dextran 75 solutions are used clinically as plasma expanders. They will also cause charge shielding and at the right concentrations induce flocculation of red cells, a trick that is used in preparing leucocyte rich plasma for white Cell purification in the laboratory. Cross linked dextran is the basis for sephadex. Commercially derived from strains of Leuconostoc mesenteroides.