The first line of defense that the body has against foreign invaders is the:

• A. Hair
• B. Skin
• C. Neutrophils
• D. Primary lymphoid tissue

Answer: (B)

The first line of defense is the skin. It serves as a continuous physical barrier that prevents microbes from entering the body; its keratinized outer layer and intact integrity create a difficult entry point. Hair is part of the skin and can help trap debris, but it isn’t the primary barrier to infection. Neutrophils are white blood cells that respond once a breach occurs, forming part of the second line of defense through phagocytosis and inflammation. Primary lymphoid tissue (like bone marrow and thymus) is involved in producing immune cells, not blocking entry. So the skin is the best answer because it is the body's first, frontline barrier against invaders.