What is a vaccine defined as?

A vaccine is defined as a suspension of killed or modified microorganisms that stimulate an immune response. This definition captures the essential purpose and mechanism of vaccines, which is to prepare the immune system to recognize and fight off specific pathogens, such as viruses or bacteria, without causing the disease itself.

Vaccines often contain antigens derived from pathogens that have been inactivated or attenuated, allowing the body to mount an immune response. This immune response can include the production of antibodies, which are crucial for protecting against future infections by the actual pathogen. By introducing a harmless component of the microorganism, vaccines effectively train the immune system, enhancing its ability to respond swiftly and effectively if exposed to the real pathogen in the future.

The other options do not accurately describe a vaccine's function. Swelling and inflammation are signs of an immune response but are not characteristics of a vaccine itself. Increased susceptibility to disease and lack of resistance indicate a weakened immune response or condition, which contrasts with what a vaccine aims to achieve.