Which of the following is an example of passive immunity?

Injection of preformed antibodies

The correct answer is Option (2) - Injection of preformed antibodies

Passive immunity refers to the transfer of preformed antibodies from one individual to another. This can occur naturally or artificially. An example of passive immunity is when preformed antibodies are directly injected into an individual to provide immediate protection against a specific pathogen or toxin.

The injection of preformed antibodies, also known as passive immunization, bypasses the need for the recipient's immune system to produce its own antibodies. These antibodies can be obtained from a donor who has already developed immunity to the target antigen through prior exposure or vaccination. The transferred antibodies can quickly neutralize the pathogen or toxin, providing temporary protection to the recipient.

Passive immunization is particularly useful in situations where immediate protection is needed or when the recipient's immune system is unable to produce an adequate immune response on its own. It is commonly used in cases of snakebites, certain viral infections, and prevention of certain diseases in individuals with compromised immune systems.

In contrast, active immunity, which is achieved through vaccination or natural exposure to antigens, involves the activation of the recipient's immune system to produce its own antibodies and memory B and T-cells. Active immunity provides long-term protection as the immune system retains a memory of the encountered antigens, allowing for a rapid and specific immune response upon subsequent exposure.

Therefore, the production of memory B and T-cells is associated with active immunity, while the injection of preformed antibodies exemplifies passive immunity.