Which of the following is a form of nonspecific defense?

Nonspecific defense mechanisms are those that provide immediate protection against a wide range of pathogens without targeting specific antigens. Natural killer cells play a crucial role in the innate immune response by monitoring the body's cells and responding to infected or cancerous cells. They do not require prior exposure to a pathogen, which distinguishes them from other immune responses that are more specific.

Antibody-mediated immunity and cell-mediated immunity, in contrast, represent specific defense mechanisms. These processes involve the activation of lymphocytes and the production of antibodies, both of which are tailored to respond to specific pathogens based on previous encounters. Therefore, while these forms of immunity are essential for a robust immune response, they do not fall under the category of nonspecific defenses like natural killer cells do.