Final answer:
Recruited leukocytes that have invaded the vessel wall are directed to the pathogen by the process of chemotaxis, where they follow chemical signals to the site of infection or injury.
Step-by-step explanation:
The process by which leukocytes are directed to the pathogen after invading the vessel wall is called chemotaxis. After leukocytes undergo diapedesis, or emigration, where they squeeze through the endothelial cells of blood vessel walls to enter the tissues, they then move toward the site of infection or injury. This movement is guided by chemical signals in a process known as positive chemotaxis—a kind of chemical '911' call from injured or infected cells that attracts more leukocytes to the site of the pathogen.