Which term describes the body's defenses that are present from birth, such as skin and mucus?

• A. Adaptive immune system
• B. Antibody
• C. Antigen
• D. Innate immune system

Answer: (D)

From birth, the body's first-line defenses are non-specific barriers that protect against many pathogens. This is the innate immune system. It includes physical barriers like the skin and mucus membranes that block entry, as well as chemical and cellular defenses such as enzymes in saliva and stomach acid, and phagocytes that swallow invaders. These responses are rapid and do not depend on prior exposure, making them non-specific.

The adaptive immune system, by contrast, develops after exposure to a specific pathogen and creates tailored responses, including antibodies produced by B cells and specialized T cells. An antibody is part of that targeted defense, while an antigen is any substance that can trigger an immune response.