Final answer:
The human body has three lines of defense to protect against pathogens. The first line includes physical and chemical barriers, the second line involves immune cells and inflammation, and the third line is the immune response.
Step-by-step explanation:
Lines of Defense in the Human Body
The human body has three lines of defense to protect against pathogens or disease-causing agents. The first line of defense includes physical and chemical barriers that keep most pathogens out. For example, the skin acts as a physical barrier, while tears and stomach acid produce chemicals that can kill pathogens.
If pathogens manage to break through this first line of defense, the second line of defense is activated, which includes immune cells and inflammation. Finally, if the pathogens manage to evade the first two lines of defense, the third line of defense, known as the immune response, kicks in, which involves specific responses to different types of pathogens.