A leukocyte, commonly known as a white blood cell, is a crucial component of the immune system. These cells help the body fight infections and diseases by identifying and attacking pathogens like bacteria and viruses. There are several types of leukocytes, including neutrophils, lymphocytes, and monocytes, each with specific functions in immune response. Leukocytes are produced in the bone marrow and can be found in the bloodstream and lymphatic system. Their levels can increase in response to infections or inflammation, serving as an indicator of the body's immune activity. Monitoring leukocyte counts is important in diagnosing various medical conditions.