Which of the following is considered the body's first line of defense?

The body's first line of defense against pathogens is the skin and mucous membranes. These structures serve as physical and chemical barriers that help prevent the entry of microorganisms and other harmful agents into the body. The skin is the largest organ and acts as a formidable barrier due to its tough, protective outer layer. It also produces antimicrobial substances that can neutralize threats.

Mucous membranes line the openings of the body, such as the respiratory, gastrointestinal, and urogenital tracts. They produce mucus, which traps pathogens and particles, and contain cilia that help move invaders away from internal tissues. This is part of an innate immune response that acts quickly and non-specifically to defend the body.

In contrast, white blood cells, antibodies, and fever responses are components of the immune system that become activated after pathogens have breached the first line of defense. While essential in combating infections, these elements operate further along the immune response timeline, rather than serving as an initial protective barrier.