What constitutes the first line of defense against pathogens in the body?

The first line of defense against pathogens in the body primarily includes physical and chemical barriers that prevent the entry of pathogens. Among the options provided, the skin serves as a critical component of this defense system. It acts as a physical barrier, protecting underlying tissues from infection, and it also produces antimicrobial substances that help inhibit pathogen growth.

While the nose and mucus play a role in trapping pathogens entering through the respiratory system, the skin is generally considered the most important initial barrier for the entire body. Mucous membranes also serve as a line of defense by trapping pathogens and producing mucus to eliminate them, but the skin covers the entire body surface and is the most significant first responder to potential threats.

Thus, the skin is recognized as the primary line of defense against pathogens due to its comprehensive protective functions.