The cellular distribution of MHC class I versus MHC class II molecules is quite different, with MHC class II molecules generally expressed on a very limited set of cell types. This is because:

Correct Answer: B

Rationale: MHC II on APCs (macrophages, B cells) allows CD4 T-cell help ; CD8 killing or DC focus aren't the reason.

Correct Answer: D

Rationale: The lymphatic system returns interstitial fluid to blood and combats infections via lymph nodes and organs, not coagulation or hormone production.

Correct Answer: A

Rationale: Interleukin-2, from helper T cells, promotes T cell proliferation, not mast cell histamine release, APC stimulation, or macrophage phagocytosis.

Correct Answer: C

Rationale: C3b coats pathogens to enhance phagocytosis (opsonization), while antibodies also opsonize, but surfactants do not; C3b is the most specific example here.

Correct Answer: B

Rationale: BCRs have a transmembrane tail, while secreted antibodies lack it, retaining two binding sites.