Practice question

Polysaccharide vaccines face intrinsic limitation that they induce serotype specific immunity due to structural diversity of capsular polysaccharides among bacterial strains. For instance Streptococcus pneumoniae possesses greater than 90 serotypes each with distinct repeating unit composition differences in sugar sequence, glycosidic linkage alpha 1-3 vs beta 1-4, side chains and O-acetylation patterns dictating distinct antigenic epitopes. Antib Antibodies raised against polysaccharide of serotype 14 recognizing beta-D-galactose 1-4 linkage do not cross react with serotype 19F containing glucose rhamnose, providing no protection against heterologous type. Consequently pneumococcal polysaccharide vaccine PPSV23 covers 23 most prevalent serotypes but still leaves coverage gap and replacement by non vaccine serotypes observed post introduction due to ecological niche filling. Similarly meningococcal polysaccharide A,C,W,Y required separately, Haemophilus type b polysaccharide PRP not cross reacting with other H. influenzae types. T-independent nature compounds limitation producing IgM and IgG2 without memory, ineffective in infants. Conjugate technology partially overcomes but serotype coverage still finite, necessitating continual surveillance to update formulation based on serotype epidemiology and antimicrobial resistance patterns.