Which system fights off disease?
Your immune system.
The human immune system is the system that fights off disease. When a pathogen invades your body, your immune system will send special cells to identify the pathogen. If you've been infected with that illness before, your body will send specialized cells that were specifically made to fight that illness. After all the cells pathogens are destroyed, phagocytes engulf the remains of the "battle" and enter the blood stream as waste.