What constitutes the body's first line of defense against disease?

The body's first line of defense against disease is indeed intact skin and mucous membranes. This outer barrier plays a critical role in preventing pathogens such as bacteria, viruses, and fungi from entering the body. Skin acts as a physical shield, while mucous membranes produce mucus that traps pathogens. Additionally, these membranes can contain antimicrobial substances that further inhibit the growth of harmful microorganisms.

This mechanism is fundamental in the immune response, providing an immediate barrier without the need for the body to mount a specific immune response. The intact skin is effective due to its structural integrity and the presence of skin flora that can outcompete harmful organisms. Mucous membranes line various body cavities and serve similar protective roles in areas such as the respiratory, gastrointestinal, and urogenital tracts.

While other options, like inflammation, antibodies in blood, and phagocytes, contribute significantly to the immune system's function, they represent subsequent lines of defense that become activated after pathogens bypass the initial barrier provided by the skin and mucous membranes.