Final answer:
Surface barriers such as skin and mucous membranes act as the immune system's first line of defense, protecting the body against pathogens.
Step-by-step explanation:
Surface barriers like cutaneous and mucous membranes serve the immune system as the first line of defense. These barriers are comparable to a castle's moat and walls, providing a non-specific defense that attempts to keep all pathogens at bay. This first line of defense includes the skin, which is a mechanical barrier, along with chemical barriers like stomach acid and biological barriers like friendly bacteria. If pathogens manage to break through this first line, the body's second line of defense is then activated.