Final answer:
Active immunity is an immune response in which B cells transform into plasma cells and secrete antibodies to fight against a pathogen.
Step-by-step explanation:
An immune response in which B cells transform into plasma cells and secrete antibodies is known as active immunity. When a pathogen invades the body and activates the immune system, B cells are activated and differentiate into plasma cells. These plasma cells produce and secrete antibodies to fight against the pathogen. The antibodies bind to the antigens on the surface of the pathogen, marking it for destruction by other immune cells.