Which of the following is considered a second line of defense in the human immune system?

• A. Mucous membranes
• B. Production of antibodies
• C. Phagocytic white blood cells
• D. Skin barrier

Answer: (C)

Verified step by step guidance

1. Understand the concept of the human immune system's lines of defense: The first line includes physical and chemical barriers that prevent pathogen entry, the second line involves internal cellular and chemical defenses, and the third line is the adaptive immune response.
2. Identify the first line of defense components: These include physical barriers like the skin and mucous membranes that block pathogens from entering the body.
3. Recognize the second line of defense: This consists of non-specific immune responses such as phagocytic white blood cells (e.g., macrophages and neutrophils), inflammation, fever, and antimicrobial proteins that act once pathogens bypass the first line.
4. Understand the third line of defense: This is the adaptive immune response involving the production of antibodies by B cells, which target specific pathogens.
5. Match each option to the correct line of defense: Mucous membranes and skin barrier belong to the first line, phagocytic white blood cells belong to the second line, and production of antibodies belongs to the third line.