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

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.

Permanent incorporation of antibodies into the host genome which causes exponential increase rather than decline of antibody levels over time.

Transformation of antibodies into infectious particles that are excreted in the urine, accounting for the rapid and sudden disappearance of passive immunity within hours.

Catabolism of antibodies through normal protein turnover pathways, including Fc receptor-mediated recycling and proteolytic degradation, leading to a decay in circulating antibody concentration.