The purpose of the system is to prevent foreign substances from entering the body and to attack foreign substances that are in the body.
A. immune
B. endocrine
C. nervous
D. excretory

Immune

Explanation: The immune system is the body's defense against infectious organisms and other invaders. Through a series of steps called the immune response, the immune system attacks organisms and substances that invade body systems and cause disease. The immune system is made up of a network of cells, tissues, and organs that work together to protect the body.