Passive immunity depends on temporary antibodies

Steps:

• Recall passive immunity involves receiving pre-formed antibodies from another source, like maternal milk or injections.
• Understand these antibodies provide immediate protection but do not stimulate the recipient's own immune response.
• Note that the body naturally degrades foreign antibodies over time, limiting duration.
• Compare to active immunity, where the body produces lasting memory cells.

Why A is correct:

• Antibodies have a half-life of about 21 days and are broken down by the body's catabolic processes, as defined in immunology.

Why the others are wrong:

• B: Antigens trigger immunity but are not the focus of passive protection's short duration.
• C: Memory cells form in active immunity, not passive, so their death is irrelevant.
• D: Phagocytes are innate immune cells that persist and are not affected in passive immunity.

Final answer: A