A preparation of microbial antigens that provokes an immune response (i.e. the production of antibodies) on injection, thus conferring immunity on the recipient. There are three types of vaccines:1.Those containing material from a nonvirulent organism that retains its immunogenicity but does not result in infection.2.Those containing a modified toxin (a toxoid) that has lost its toxic properties but retains its immunogenicity.3.Those containing live, attenuated organisms (i.e. genetic variants of a virus or bacterium) that are antigenically similar to the original strain but lack virulence.Recombinant DNA research has allowed the production of new and more specific vaccines. For example, the gene for the B antigen of hepatitis virus has been cloned in E. coli, the protein expressed and a specific anti-B antiserum produced which can be used as a vaccine.