How does the skin function as a barrier to infection?

The skin serves as a critical barrier to infection primarily by providing both a physical and chemical defense against pathogens. As the body's first line of defense, its outermost layer, the epidermis, acts as a physical barrier that prevents microorganisms from entering the body. This barrier is composed of tightly packed cells that are difficult for pathogens to penetrate.

In addition to this physical barrier, the skin also produces antimicrobial substances, such as defensins and lysozymes, which help to neutralize or destroy potential pathogens that come into contact with the skin’s surface. These substances contribute to the skin's role in the immune response, inhibiting the growth of harmful bacteria and fungi.

The other choices do not accurately describe the primary functions of the skin in relation to infection. Skin does not absorb pathogens; rather, it prevents their entry. While the skin does support beneficial bacteria, the secretion of nutrients is not its primary function concerning infection control. Furthermore, while the respiratory system does involve surfaces for gas exchange, the skin is not primarily engaged in respiratory functions, making it incorrect in this context. Thus, the combination of a physical barrier and antimicrobial properties firmly establishes the skin's role in protecting against infections.