Why is passive immunity of much shorter time or duration than active immunity?

Passive immunity is of shorter duration compared to active immunity due to the following reasons:

Type of antibodies: In passive immunity, ready-made antibodies are obtained from an external source, such as through the injection of immune serum or immunoglobulin preparations. These antibodies are not produced by the recipient's own immune system.

Antibody turnover: Antibodies have a natural lifespan and undergo degradation and elimination from the body over time. Since the antibodies in passive immunity come from an external source, they are not continuously produced or replenished by the recipient's immune system. As these antibodies are gradually broken down and removed from the body, the protection provided by passive immunity diminishes.

Immune memory: Active immunity involves the activation of the recipient's own immune system, leading to the production of memory cells. These memory cells remain in the body after the infection or immunization and can rapidly respond to future encounters with the same pathogen. This allows for a faster and more robust immune response upon re-exposure, resulting in long-term protection.

In contrast, passive immunity does not stimulate the recipient's immune system to generate its own memory cells. Therefore, when the passively acquired antibodies are depleted, the recipient loses the protection and becomes susceptible to the infection again.

Duration of protection: The duration of passive immunity typically lasts for a limited period, ranging from a few weeks to several months. This is determined by the half-life of the antibodies obtained from the external source and their rate of clearance from the recipient's body.

For these reasons, passive immunity provides temporary protection but does not offer the same level of long-lasting immunity as active immunization, where the recipient's immune system actively participates in the defense against pathogens.