With the exception of basophils, the polymorphonuclear leukocytes (PMNs) or granulocytes from human peripheral blood have densities predominantly above 1.080 g/ml, while mononuclear cells (MCs) have densities below 1.077 g/ml (see Figure 1). 

Since the density of erythrocytes significantly overlaps that of the denser neutrophils there is only one means by which PMNs may be isolated from whole human blood using a single step method and that is to use the Axis-Shield medium called Polymorphprep™. It contains diatrizoate and a polysaccharide [1] and has a high density (1.113 g/ml) and osmolality (445 mOsm).

 The high osmolality causes loss of water from the erythrocytes; creating a continuous gradient in which the MCs and PMNs band according to their buoyant densities