when spleen Contract, circulation is closed, while when spleen relax. Circulation open

Correct Answer: B

Rationale: Open-closed theory describes spleen circulation: closed when contracted, open when relaxed.

Correct Answer: D

Rationale: Context suggests thymus (CD4+ and CD3+ T-cell production, D); spleen degrades RBCs , lymph nodes have vessels , tonsils produce antibodies .

Correct Answer: C

Rationale: Cytokines from macrophages signal and coordinate immune responses ; they don't destroy pathogens directly , form vesicles , or primarily affect blood flow .

Correct Answer: C

Rationale: Co-stimulatory molecules (e.g., B7) bind CD28 on T cells for activation ; pattern recognition , DC activation , and killing are distinct.

Correct Answer: B

Rationale: CTLA-4 inhibits T-cell activation by competing with CD28 ; it doesn't activate (A, C) or target B7 directly .