What is the primary reason that the integumentary system, including skin and hair, is considered the first line of defense against pathogens?

The skin and hair form a physical barrier that stops germs from entering the body, so they act as a shield before any immune cells get involved. Their tight cells and a dry, acidic surface make it hard for bacteria and viruses to survive or cross into deeper tissues. The skin’s outer layer also releases antimicrobial peptides that kill microbes that touch it. Because this barrier works immediately without needing a previous infection, it is the first line of defense. For example, a cut on the skin allows bacteria to enter, showing how the intact skin prevents infection.