Certified Nursing Assistant (CNA) Practice Exam

The skin and mucous membranes serve as the body's first line of defense against infection due to their barrier functions. The skin provides a physical barrier that protects underlying tissues from pathogens, while mucous membranes, found in areas such as the respiratory and digestive tracts, trap and filter out harmful microorganisms and particles. Additionally, both the skin and mucous membranes produce antimicrobial substances that help to neutralize potential threats.

The digestive system, circulatory system, and skeletal system all play important roles in overall health and immune response, but they do not primarily act as the first means of protection against infections. The digestive system helps process food and can deal with ingested pathogens, while the circulatory system transports immune cells throughout the body, and the skeletal system supports structure and aids in the production of blood cells. However, they do not provide the immediate physical barrier that the skin and mucous membranes do.