Sequence of Events in Primary Immune Response

Steps:

• T-helper cell binds antigen via complementary receptor (4), starting recognition.
• Macrophage phagocytoses microbe and presents antigen (1), confirming and processing the signal.
• T-helper divides by mitosis into clones (3), expanding the response.
• Clones differentiate into T-killer cells targeting infected cells (2), executing defense.

Why B is correct:

• Matches the immunological sequence where T-cell binding initiates activation before full phagocytosis and clonal expansion, per adaptive immunity principles.

Why the others are wrong:

• A: Places division (3) before binding (4), skipping activation step.
• C: Begins with unstimulated division (3), violating cell cycle rules.
• D: Starts with division (3) and ends with presentation (1), inverting antigen processing order.

Final answer: B