Which cells of the second line of defense recognize and kill virus-infected cells?

• A. Basophils
• B. Eosinophils
• C. Natural killer (NK) cells
• D. B lymphocytes

Answer: (C)

Verified step by step guidance

1. Understand that the second line of defense in the immune system is part of the innate immune response, which acts quickly to fight infections without prior exposure.
2. Identify the main cell types involved in the second line of defense: these include phagocytes (like macrophages and neutrophils), natural killer (NK) cells, and other innate immune cells.
3. Recall that Natural Killer (NK) cells are specialized lymphocytes that recognize and kill virus-infected cells by detecting changes in the infected cells, such as reduced MHC class I molecules on their surface.
4. Note that basophils and eosinophils are primarily involved in allergic responses and defense against parasites, not directly in killing virus-infected cells.
5. Understand that B lymphocytes belong to the adaptive immune system (third line of defense) and produce antibodies, so they are not part of the innate second line of defense that kills virus-infected cells.