Final answer:
Diapedesis, also known as emigration, is the process where white blood cells attach to endothelium and squeeze through vessel walls in response to chemical signals from damaged or infected cells.
Step-by-step explanation:
The phenomenon where white blood cells (WBCs) line up at the capillary walls and attach to the edge of the endothelium is called diapedesis, also known as emigration. During this process, the leukocytes squeeze through adjacent cells in a blood vessel wall to enter tissues. This occurs in response to positive chemotaxis, a situation where injured or infected cells and nearby leukocytes emit chemical signals that attract more WBCs to the site of damage or infection.