Phagocytes engulf pathogens in primary immune response

Steps:

• Identify phagocytes as cells that engulf and destroy pathogens via phagocytosis in innate immunity.
• Recall primary immune response starts with innate cells like monocytes, which mature into macrophages.
• Eliminate options: plasma cells produce antibodies, T-helper cells activate others, T-killer cells lyse infected cells.
• Confirm monocytes as key phagocytes differentiating into tissue macrophages.

Why A is correct:

• Monocytes are phagocytes that ingest pathogens, per their role in innate immunity definition.

Why the others are wrong:

• B: Plasma cells secrete antibodies, not phagocytose.
• C: T-helper cells coordinate immune responses via cytokines, not phagocytosis.
• D: T-killer cells destroy infected cells by inducing apoptosis, not engulfing.

Final answer: A