Answer:
The correct answer is D. is a system that differentiates self from nonself to neutralize harmful invaders.
Step-by-step explanation:
A defense system that protects the host body from pathogen infection is called the immune system. This immune system is made up of many different barriers which helps in preventing illness from pathogen attack.
Physical and chemical barriers provide the first line of defense to the body for example skin, mucous membrane, lysozyme in saliva provide first line of defence. Second line of defense is provided by non-specific immune cells like mast cells, macrophages, dendritic cells, etc.
The third line of defense is provided by pathogen-specific immune cells like T lymphocytes and B lymphocytes. These cells have the ability to differentiate self and non-self cell and kills the non-self cell that come from outside the body.