Final answer:
The internal environment of the human body is protected by the innate immune system, which includes physical and chemical barriers like the skin, tears, and mucus. These barriers serve as the first line of defense against pathogens.
Step-by-step explanation:
The internal environment of the human body is protected by the innate immune system, which is referred to as the first line of defense. This defense includes both physical and chemical barriers that are always ready to defend the body from infection. Being the "first line", it starts with the skin, which acts like the moat and walls of a castle, providing a barrier that pathogens must overcome to cause harm.
Other components of the body's first line of defense include tears, mucus, cilia, stomach acid, urine flow, and friendly bacteria. These barriers operate continuously, not just in response to infections, and they protect against a broad range of pathogens by preventing them from entering the body, destroying them after they enter, or flushing them out before they can establish themselves.
If pathogens do manage to breach these initial defenses, the body's second line of defense is activated, which includes a variety of specialized cells and factors that mount a non-specific immune response to invaders.