What serves as the body’s first line of defense against infection?

The body's first line of defense against infection is primarily the unbroken skin. This outer barrier plays a crucial role in protecting internal tissues from pathogens, such as bacteria and viruses. The skin is composed of multiple layers that act as a formidable physical barrier, preventing the entry of harmful microorganisms. Its integrity is vital; when the skin is intact, it inhibits pathogens from penetrating deeper and causing infections.

Mucous membranes, while also important in defense and providing a barrier in areas like the respiratory and gastrointestinal tracts, are considered a secondary line of defense. They produce mucus that traps pathogens and contain various antimicrobial substances, but they do not offer the same level of physical protection as unbroken skin.

White blood cells and antibodies are essential for the immune response once an infection has occurred, providing a targeted attack against pathogens. However, they work after the initial barriers have been breached. Hence, while these components are vital for the body's overall immune function, they do not constitute the first line of defense in the same way that unbroken skin does.