What does a vaccine contain?

A vaccine contains a killed or weakened strain or critical antigens from a pathogen to stimulate the immune system without causing the disease itself. This exposure allows the immune system to recognize and fight the actual pathogen if encountered in the future. The use of either killed or attenuated (weakened) forms of the pathogen or specific components such as proteins or polysaccharides that represent the pathogen’s structure essentially educates the immune system. This process leads to the development of a memory response, enabling the body to mount a rapid and effective defense against subsequent infections.

The other choices suggest forms of the pathogen that could potentially cause disease, such as a living pathogen or a full-strength version which would not be safe or effective as a vaccine. The focus is on creating immunity without triggering the illness, ensuring safety while establishing a robust immune response. Therefore, the presence of safe components derived from pathogens, as provided in option C, is crucial for the development of effective vaccines.