Antigens circulate in blood plasma

Steps:

• Define antigen as a foreign molecule triggering immune response.
• Recall antigens include soluble proteins or pathogens in bodily fluids.
• Identify blood plasma as a key fluid where antigens are detected and circulate.
• Eliminate options by checking if antigens are surface-bound or soluble.

Why B is correct:

• Antigens are defined as substances in blood plasma that bind antibodies and activate immune cells, per immunology basics.

Why the others are wrong:

• A: Macrophages present antigens via MHC molecules but do not inherently have them on their surface.
• C: Antigens exist in plasma, so "neither" ignores this fact.
• D: Antigens are primarily soluble in plasma, not fixed on macrophage surfaces.

Final answer: B