The first line of defense of innate immunity includes ______.

• A. complement system activation
• B. antibody production by B cells
• C. physical and chemical barriers such as skin and mucous membranes
• D. activation of cytotoxic T lymphocytes

Answer: (C)

Verified step by step guidance

1. Understand that the innate immune system is the body's first line of defense against pathogens and acts immediately or within hours of an antigen's appearance in the body.
2. Recall that the first line of defense primarily consists of physical and chemical barriers that prevent pathogen entry, such as the skin and mucous membranes.
3. Recognize that complement system activation and cytotoxic T lymphocyte activation are part of later immune responses, not the initial physical and chemical barriers.
4. Note that antibody production by B cells is part of the adaptive immune response, which is slower and more specific than innate immunity.
5. Conclude that the first line of defense in innate immunity includes physical and chemical barriers like skin and mucous membranes, which act as a non-specific protective shield.