Antibodies circulate in body fluids during immune responses

Steps:

• Recall antibodies are proteins produced by B cells in humoral immunity.
• Note they are secreted by plasma cells into extracellular spaces.
• Identify primary locations as blood, lymph, and mucosal secretions.
• Confirm they target pathogens in fluids, not inside cells.

Why A is correct:

• Antibodies are soluble immunoglobulins that circulate freely in plasma and other body fluids, enabling humoral immune responses as defined in immunology.

Why the others are wrong:

• B: Antibodies do not enter cells; they act extracellularly against pathogens.
• C: Secreted antibodies are widespread, not confined to B cell surfaces.
• D: Antibodies function in extracellular environments, not intracellular compartments.

Final answer: A