Final answer:
The body's first line of defense is a combination of physical barriers, with the skin being the most important. These barriers, including mucous membranes and body fluids, protect against the invasion of pathogens.
Step-by-step explanation:
Your body's first line of defense is a simple physical barrier, which includes the skin, among other mechanisms. The skin acts much like a castle's moat and walls, keeping most pathogens out of your body. This defense is non-specific, meaning its primary goal is to protect against all types of pathogens indiscriminately. Besides the skin, this line of defense also comprises tears, mucus, cilia, stomach acid, urine flow, and friendly bacteria.
The innate immune system first uses physical and chemical barriers like the skin, mucous membranes, and substances in body fluids to prevent infection. The skin is particularly important, as its outer layer, the epidermis, is tough and very difficult for pathogens to penetrate. It consists of dead cells that are constantly being shed, helping to remove infectious agents. In addition to these barriers, hairs in our nose and elsewhere act as traps for larger particles and pathogens.
The body's physical defenses are designed to keep pathogens out or to flush them away before they can establish themselves. These defenses always stand ready to protect the body, as a non-responsive mechanism to potential pathogen invasion. They work continuously, unlike other immune responses that are activated only after an infection has been detected.