Weakened or killed antigens purposefully introduced into the body by medical professionals is known as __________.

The correct answer is "Vaccine." A vaccine consists of weakened or killed forms of pathogens, such as viruses or bacteria, that stimulate the body’s immune response. When a vaccine is administered, it trains the immune system to recognize these antigens, so that if the body is later exposed to the actual pathogen, it can respond more effectively and efficiently, providing immunity against disease.

This process helps the body build protection without causing the disease itself. The use of vaccines has been a pivotal development in public health, significantly reducing the incidence of various infectious diseases. Other terms in the options, such as antiviral, refer to medications used to treat viral infections, and antibodies are proteins produced by the immune system in response to antigens. A pathogen, on the other hand, is a microorganism that can cause disease, which is what vaccines target but are distinct from in their purpose and function.