Which of the following are considered barriers to pathogens in the human body? Select all that apply.

Barriers that stop pathogens from entering the body include the skin, which blocks most microbes; mucous membranes that trap and move invaders out; the acidic environment of the stomach that kills many bacteria; and fluids such as tears, saliva, and mucus that contain enzymes and antibodies. Each barrier works by either physically preventing entry or by chemically destroying the pathogen before it can reach tissues. For example, the skin’s outer layer is impermeable to most germs, while the stomach’s pH of about 2 breaks down proteins of bacteria that slip past the skin. These defenses act together so that a single pathogen rarely succeeds in infecting the body.