What is the function of leukocytes?

Leukocytes have an important function in humans and many other animals. They are found in the blood fluid. Their function is to protect the body from various foreign agents that enter the body. For example, when a person is injured and an infection has entered the bloodstream, leukocytes attack the threat, saving the body. Leukocytes are able to remember viruses and bacteria and know how to deal with them next time. This is how immunity is developed. Sometimes it is permanent, and sometimes it lasts for a short time, for example – immunity to cholera.