T-Helper Cells Activate Adaptive Immunity

Steps:

• T-helper cells (CD4+ T cells) recognize antigens and release cytokines like IL-2 and IL-4.
• These cytokines promote proliferation of B-lymphocytes for antibody production.
• The same cytokines stimulate cytotoxic T-killer cells (CD8+ T cells) to expand and target infected cells.
• Thus, both B and T-killer cells divide in response, not just one type.

Why D is correct:

• T-helper cytokines, per immunology definitions, specifically induce clonal expansion of both B-lymphocytes (humoral response) and cytotoxic T cells (cell-mediated response).

Why the others are wrong:

• A: Macrophages are activated for phagocytosis by cytokines but not primarily stimulated to divide.
• B: Excludes T-killer cells, which also proliferate via IL-2 from T-helpers.
• C: Excludes B-lymphocytes, which require T-helper signals for activation and division.

Final answer: D