Streptococcus pneumoniae Capsular Polysaccharide Antigen (CPS)

The *Streptococcus pneumoniae* capsular polysaccharide antigen is a complex carbohydrate structure that forms the outermost layer (capsule) of encapsulated strains of *S. pneumoniae*. This capsule is composed entirely of polysaccharides and was the first non-protein antigen to be recognized in bacteria. The capsular polysaccharide is the dominant surface structure and serves as a critical virulence factor for this pathogen. It protects *S. pneumoniae* from host immune responses, primarily by interfering with opsonophagocytic clearance mechanisms. There are currently 98 recognized serotypes of *S. pneumoniae*, each defined by unique structural variations in their capsular polysaccharides. The capsular polysaccharide antigens form the basis for both plain-polysaccharide vaccines (such as PPSV23) and conjugate vaccines (such as PCV13) to elicit protective antibody responses. Immunity conferred by these vaccines is strictly serotype-specific due to structural differences between capsules; thus, vaccine coverage depends on included serotypes matching those circulating in populations. Widespread vaccination has altered epidemiology by reducing vaccine-type disease but increasing prevalence of non-vaccine types.

Other names

Streptococcus pneumoniae type X capsular polysaccharide antigenPneumococcal capsular polysaccharidePneumococcus capsule antigen

Antibody generation; Opsonophagocytosis