T-lymphocytes' roles in adaptive immunity

Steps:

• Identify T-cell types: helper T cells produce cytokines to coordinate immune responses.
• Note cytotoxic T cells release perforin and granzymes, which act as toxins to destroy infected cells.
• Confirm T cells recognize antigens presented by MHC molecules on antigen-presenting cells (APCs) via T-cell receptors.
• Evaluate statements: all three (1, 2, 3) align with established T-cell functions.

Why A is correct:

• T-lymphocytes perform all listed functions, as defined in immunology: cytokine secretion by CD4+ cells, toxin-like molecules by CD8+ cells, and antigen recognition on APCs.

Why the others are wrong:

• B omits antigen recognition, essential for T-cell activation.
• C excludes toxin production, a key cytotoxic function.
• D excludes cytokine production, critical for immune regulation.

Final answer: A