Final answer:
Leukocytes are the components of the immune system responsible for fighting off bacteria, fungi, and viruses. This is part of the adaptive immune system which has a memory to respond faster to known pathogens.
Step-by-step explanation:
The system that uses leukocytes to combat bacteria, fungi, and viruses is the immune system. Leukocytes, also known as white blood cells, are a crucial component of the immune system. They are produced by the body as a natural response to an infection. The vertebrate, including human, immune system is a complex multilayered system for defending against external and internal threats to the integrity of the body. The immune system has several lines of defense, with the leukocytes being part of the more specialized adaptive immune system, which provides a highly specific response to pathogens and also has a memory system to respond faster upon re-exposure to the same pathogens.
Furthermore, the Lymphatic System is closely associated with the immune system, playing a key role in the immune response and in defending the body against disease.