The main function of the immune system is to​

A) ​manufacture white blood cells.
B) ​manufacture red blood cells.
C) ​protect internal organs.
D) ​defend against foreign invaders.

The immune system is responsible for defending the body against foreign invaders such as bacteria, viruses, and parasites. It does this through the production of specialized white blood cells and other immune molecules that target and neutralize these invaders. Protecting internal organs is an important function of the immune system, but it is secondary to its primary role in defending against foreign threats. The production of red blood cells is not a primary function of the immune system.