White Blood Cells (also known as Leukocytes) as far as I know fight off bacteria or what is considered a threat to the rest of the body. So, they usually respond to harmful bacteria within the blood stream. So I believe the specific response would be if you were to be infected or have a disease the white blood cells would respond in trying to kill off the harmful disease.
An additional note is that the white blood cells are created within the Bone Marrow.
