The ________ provides the body with its first line of defense against foreign substances.

• A. Innate immune system
• B. Lymphocyte
• C. T cell
• D. Antibody

Answer: (A)

The innate immune system provides the body's first line of defense against foreign substances. It offers immediate, non-specific protection at entry points like the skin and mucous membranes. This includes physical barriers (skin, mucous membranes), chemical barriers (acidic pH, enzymes), and cellular defenses (phagocytes such as neutrophils and macrophages) that respond quickly to a wide range of invaders. In contrast, lymphocytes, T cells, and antibodies belong to the adaptive immune system, which responds more slowly on first exposure but targets specific pathogens and can remember them for faster future responses. Because the question asks about the initial, non-specific defense, the innate immune system is the best fit.