The first line of defense against an invading pathogen is/are the
A. skin, digestive tract, respiratory tract, and urinary tract.
B. phagocytic cells, natural killer cells, complement proteins, and interferons.
C. production of cells or antibodies for a specific antigen.
D. all of the above.
The correct answer and explanation is:
Correct Answer: A. skin, digestive tract, respiratory tract, and urinary tract.
Explanation (300 words):
The immune system defends the body against invading pathogens through multiple layers of defense. These are commonly divided into three lines of defense, and the first line is the body’s physical and chemical barriers.
Option A includes the skin, digestive tract, respiratory tract, and urinary tract—all of which are part of the first line of defense.
- Skin acts as a tough, impermeable barrier that prevents entry of pathogens.
- Digestive tract contains stomach acid and enzymes that destroy harmful microbes ingested with food.
- Respiratory tract is lined with mucus and cilia that trap and sweep out pathogens.
- Urinary tract helps flush out microbes during urination, and has antimicrobial properties in urine.
Together, these barriers physically block or chemically neutralize pathogens before they can enter deeper tissues. This line of defense is nonspecific, meaning it does not target specific pathogens but rather serves as a general protective barrier.
Option B describes components of the second line of defense, which includes:
- Phagocytic cells (like macrophages) that engulf pathogens,
- Natural killer (NK) cells that destroy infected or abnormal cells,
- Complement proteins that help destroy microbes,
- Interferons that interfere with viral replication.
These components come into play after a pathogen bypasses the first line.
Option C refers to the third line of defense, which involves the adaptive immune response—production of specific antibodies and memory cells targeting particular antigens.
Option D (“all of the above”) is incorrect because the question specifically asks for the first line of defense only.
In summary, Option A is correct because it includes the primary physical and chemical barriers that first encounter and prevent the entry of pathogens into the body.