Vaccines of Defined Chemical Nature -
Since the chemical nature of many antigens is now known, it has been possible to produce these antigens either by biological or by chemical means. Perhaps the first defined molecular species to be used as a vaccine were the polysaccharide vaccines for Neisseria meningitides (meningitis) and Streptococcus pneumoniae (pneumonia).

The pure polysaccharide preparations from the bacterial capsule could be used as an antigen in adults or children of over two years of age. Addition of some other chemicals or biological principles often increases the potency of the polysaccharide antigen.

For example, Haemophilus influenzae (influenza) B polysaccharide is very poor in inducing antibodies in the susceptible host on its own. However, its antigenic property is greatly enhanced if it is mixed up with diphtheria toxoid. Proteins and nucleic acids have also been identified as immunogenic molecules.