Why is the second line of defense called that?

The second line of defense is called that because it refers to the body's immune response that activates when the first line of defense (physical and chemical barriers like skin and mucous membranes) is breached. This defense involves innate immune responses, such as inflammation and the activity of white blood cells, which act to identify and eliminate pathogens. It serves as a crucial backup system to provide immediate protection against infections and injuries.