Which of the following body systems has the primary function of fighting diseases?

The immune system is responsible for protecting the body from infections and diseases. It is a complex network of organs, cells and proteins that work together to defend the body against harmful pathogens, while also distinguishing them from the body's own healthy cells. 

The circulatory system is responsible for the transportation of blood and nutrients throughout the body, while the integumentary system protects the body from external damage. The respiratory system is responsible for the exchange of gases in the body.