Final answer:
The biological barrier, including the immune system, acts on disease-causing bacteria that make it past the first and second lines of defense.
Step-by-step explanation:
Once a disease-causing bacteria successfully enters the body past the first and second lines of defense, the biological barrier becomes the main line of defense against the bacteria. The biological barrier includes the immune system, which identifies and destroys harmful bacteria, viruses, and other pathogens.
Learn more about Bacteria invasion and the body's defense mechanisms