4.6 Summary of Section 4

• Conventional vaccines contain either killed pathogens or attenuated strains (live or killed) with the same critical antigens as the target pathogen. Genetic manipulation may delete the genes involved in pathogenicity to create new attenuated strains.

• Subunit vaccines contain pathogen fragments extracted by conventional biochemical techniques, or genetically engineered constructs. The subunit antigen may be conjugated with bacterial proteins or lipids to enhance its ability to induce protective immunity.

• DNA vaccines contain naked DNA encoding a pathogen-specific antigen which is ‘fired’ into host tissues and expressed there; vector vaccines contain pathogen genes inserted into the genomes of harmless viruses or bacteria. Both vaccine types generate an immune response when the gene product is expressed in the recipient. Oral plant vaccines are genetically engineered to express pathogen antigens with the aim of eliciting protective immunity in the gut when eaten.