How do vaccines cause immunisation to disease?

Vaccines contain an inactive form of a pathogen, or dead pathogen, and are injected into the body to give immunity to that pathogen. The inactive pathogen acts as an antigen, stimulating an immune response, and the white blood cells produce antibodies specific to the pathogen. This means if an active form of the pathogen enters the body, the white blood cells will be able to rapidly make the specific antibody that is required, and the pathogen will be destroyed. As the pathogen in the vaccine is inactive, it will not do any harm.