Conjugate vaccines and polysaccharide response
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* Sujoy Khan

Purified polysaccharide vaccines work by activation of B cells in a T-independent manner, producing predominantly IgM and little memory B cells.1 Protein-polysaccharide conjugate vaccines (such as Prevnar/PCV7 which uses 7 prevalent polysaccharides to bind to non-toxic variant of diphtheria toxin, CRM197) allow the protein to present antigen on B cells and CD40/CD40L interaction, while T cells allow antibody class switching from IgM to IgG producing memory cells and longer response. Conjugated vaccines usually use polyribosylribitol phosphate (PRP) conjugated with protein carriers and conjugate vaccines for *Haemophilus influenzae* and *Neisseria meningitidis* (using outer membrane proteins, OMP) have already been developed.2

However, conjugate vaccines may not work in high-risk categories like HIV-positive children3 and asplenics,2 and the PPV23 vaccine failure comorbid elderly4needs to be identified and followed up. IgG subclass measurement for evaluation of vaccine response is vital; anti-IgG1 pneumococcal antibodies in children (both with normal and abnormal immunity) and anti-IgG2 antibodies in adults are the best discriminatory laboratory measure.5 The future seems certain for conjugate vaccines, and PPV23 may end up being a test to see polysaccharide response in an individual.

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