Which system serves as the body's first line of defense against infection?

The integumentary system serves as the body's first line of defense against infection primarily through its physical barrier properties. This system includes the skin, hair, nails, and various glands. The skin acts as a protective layer that prevents pathogens, such as bacteria and viruses, from entering the body.

Moreover, the skin produces antimicrobial peptides and secretions, such as sweat and oils, that can inhibit the growth of microbes. Additionally, the skin is home to beneficial microorganisms that compete with potential pathogens, further enhancing its defensive role.

This system's ability to detect changes in the environment through receptors also contributes to an immediate immune response, making it an essential component of the body’s overall defense mechanisms. In comparison, the muscular, nervous, and reproductive systems do not serve this primary protective function, as they have different roles within the body.