Question

What is the function of the immune system? It exchanges oxygen and carbon dioxide. It provides structure for the body. It eliminates waste from the body. It responds when pathogens enter the body.

Answer 1

I am not 100% sure but I thinking it responds when pathogens enter the body because the immune system is meant to protect your body from illness. 

Answer 2

The function of the Immune System is to respond when pathogens enter the body.  The Immune System is not responsible for exchanging oxygen and carbon dioxide, that is the job of the Respiratory System.  It also does not provide structure for the body, that is the role of the Skeletal System.  And finally, the Excretory System is responsible for the removal of waste.