The classical complement pathway is initiated by C1q binding to the surface of a pathogen. In some cases, C1q can directly bind the pathogen, for instance by recognizing proteins of bacterial cell walls, but in most cases C1q binds to IgM antibodies that are bound to the pathogen surface. How does this IgM-binding feature of C1q contribute to rapid, innate immune responses rather than to slow, adaptive responses?

Correct Answer: B

Rationale: Pre-existing natural IgM enables rapid C1q activation; B-cell secretion is adaptive, CRP or inflammation are secondary.

Correct Answer: A

Rationale: Tropism refers to plant growth responses to stimuli (e.g., light or gravity), not branching or day/night length, which relate to other processes like photoperiodism.

Correct Answer: D

Rationale: The pancreas is not part of the lymphatic system, which includes thymus, tonsils, and spleen for immune functions.

Correct Answer: C

Rationale: Helper T cells activate B cells via cytokines, enhancing antibody production, unlike plasma cells (antibody producers), cytotoxic T cells, or macrophages.

Correct Answer: B

Rationale: C3 is central to all pathways (classical, lectin, alternative), cleaved to initiate downstream effects, unlike C9 (MAC), C1q (classical), or C4 (classical/lectin).