Cell migration.

The movement of cells around the body must be carefully controlled so that the cells go only to where they are required. This control is at two levels: one level is controlling where leukocytes leave the bloodstream; the second level is controlling where the cells go within tissues and organs once they have left the bloodstream. For a single cell, most organs are pretty big places and the cell must go to the right location within the organ or tissue.
Two important factors play an important role in controlling the movement of cells to and within specific tissue sites. Adhesion molecules are present on leucocytes and endothelial cells, and interactions between adhesion molecules allow leucocytes to bind to endothelium as part of the process of migrating across the endothelium. Chemotactic agents, especially the chemokines, are also important in controlling cell migration. They can act directly on cells and cause them to move in a particular direction or they can act indirectly by altering the expression or binding activity of adhesion molecules.