Subunit Vaccines

Subunit vaccines are used widely to protect against a number of important pathogens. Subunit vaccines typically use isolated antigenic macromolecules from the external surface of the pathogen. Vaccines against Haemophilus influenzae type b (HIb), Neisseria meningitidis and Streptococcus pneumoniae contain capsular polysaccharides derived from the bacteria. Alternatively, the vaccine may be a recombinant protein such as the hepatitis B surface antigen (HBsAg) used in the hepatitis B vaccine. Different factors influence or even dictate the use of subunit vaccines as opposed to whole organism vaccines. Gram-negative bacterial pathogens such as Haemophilus influenzae type b and Neisseria meningitidis represent a significant health threat even when dead due to the presence of lipopolysaccharide moieties (endotoxin) in their cell wall structures.

Endotoxin is a powerful inducer of proinflammatory cytokines in macrophages leading to septic shock. In the case of hepatitis B virus, the inability to culture the viruses has so far made attenuation impossible.
Despite the limitations described above, subunit vaccines are among the most efficacious vaccines due to their ability to elicit highly appropriate immune responses.