Diapedesis is the ability of white blood cells to squeeze between the cells that form blood vessel walls. Once these white blood cells are outside the blood, they move through interstitial spaces using a form of primitive movement called amoeboid motion. Neutrophils and monocytes are the most active of these white blood cells. These leukocytes engulf bacterial cells, organic molecules in bacterial cells, and other large objects such as parasites. Neutrophils and monocytes frequently become so full of bacterial toxins and other related products that they also die.