What does a vaccine consist of ? How does a vaccine work?

Vaccine consist of dead or killed or inactivated or live versions of disease-causing bacteria, virus or toxin. These are also known as a vaccine antigen. Both killed and live vaccines work by stimulating the immune system so it thinks it's being attacked by the active germ. It is a biological preparation that provides active acquired immunity to a particular disease. White blood cells – APCs, B cells, and T-cells – recognize, destroy and “remember” this version of the pathogen. That way, the immune system can quickly recognize and destroy this harmful microorganism later on. A vaccine is essentially a pathogen-imposter.