What is the primary biological mechanism responsible for the decline of passively transferred antibodies in a recipient over time?

• A. Antibody conversion into memory B cells that remain dormant and are not measurable in circulation, which is why serum antibody levels fall to zero but immunity remains intact.
• B. Permanent incorporation of antibodies into the host genome which causes exponential increase rather than decline of antibody levels over time.
• C. Transformation of antibodies into infectious particles that are excreted in the urine, accounting for the rapid and sudden disappearance of passive immunity within hours.
• D. Catabolism of antibodies through normal protein turnover pathways, including Fc receptor-mediated recycling and proteolytic degradation, leading to a decay in circulating antibody concentration.

Answer: (D)