Sequence of primary immune response steps

Steps:

• Q occurs first: T-helper cells interact with macrophages presenting digested pathogen antigens, activating T-helper cells.
• S occurs second: Activated T-helper cells release cytokines to activate B-lymphocytes and other immune cells.
• P occurs third: Activated B-lymphocytes undergo clonal expansion via mitosis to produce many identical cells.
• R occurs fourth: Some divided B-lymphocytes differentiate into plasma cells that secrete antibodies.

Why A is correct:

• B-lymphocyte division (P) follows T-helper activation of B cells (S) and precedes plasma cell differentiation for antibody production (R), per adaptive immunity clonal selection.

Why the others are wrong:

• B: Q initiates the response by activating T-helper cells via antigen presentation.
• C: R is the final effector step, occurring after B-cell proliferation.
• D: S happens early, as T-helper cells activate lymphocytes post their own activation.

Final answer: A