The system of the human body that has the primary function to fight against diseases is the immune system.

The immune system is a complex network of cells, tissues, and organs that work together to defend the body against harmful invaders such as bacteria, viruses, fungi, and parasites. These invaders are collectively known as pathogens, and they can cause a wide range of diseases.

The immune system's main functions include:

• Recognizing pathogens: The immune system must be able to distinguish between the body's own cells and foreign invaders.
• Activating immune cells: Once a pathogen is detected, the immune system activates various types of immune cells to fight it off.
• Eliminating pathogens: Immune cells work to kill or neutralize pathogens through various mechanisms.
• Remembering pathogens: After an infection is cleared, the immune system retains a memory of the pathogen, allowing it to respond more quickly and effectively if the same pathogen is encountered again in the future.

You can learn more about the immune system from reputable sources such as:

• National Institute of Allergy and Infectious Diseases (NIAID)
• National Center for Biotechnology Information (NCBI)