You see pus at the site of infection on your skin. How is it formed?
Answer: Formation of pus: Pus are nothing but a rich protein fluid and dead leukocytes (neutrophills). WBC fight foreign invaders that inter to the body. When they find disease causing bacteria in any of the body. They send message to other WBC to attract them to fight against these bacteria.
During the fight some of the WBC will die and their body will start piling up forming a wall between the bacteria and the rest of the body. These dead WBC along with some proteins of the body form a yellowish white substance called pus. So even in dead form, they will still help to protect the body from bacterial invasion.