Capsular polysaccharide

A polymer-rich outer layer surrounding some bacterial cells, often composed mainly of sugars. Its structure can protect bacteria and shape how the immune system recognizes them.

Phagocytosis: The cellular process of engulfing and internalizing particles, including microbes. A capsule can make bacterial cells harder for immune cells to engulf.
Polysaccharide: A carbohydrate polymer made of many sugar units joined by glycosidic bonds. Capsular polysaccharides are a specialized class of these large carbohydrate polymers.
Pneumococcal vaccine: A vaccine that protects against disease caused by Streptococcus pneumoniae. Many pneumococcal vaccines target capsule sugars from selected serotypes.
Streptococcus pneumoniae: A bacterial species that can cause pneumonia, meningitis, and other infections. Its capsule is a major virulence factor, and capsule types guide vaccine coverage.
Poly-γ-glutamic acid capsule: A bacterial capsule composed of poly-γ-glutamic acid rather than a carbohydrate polymer. Bacillus anthracis illustrates that bacterial capsules need not be polysaccharides.
Opsonization: The coating of a target with molecules that promote its recognition and uptake by phagocytes. Antibodies and complement can mark encapsulated bacteria for clearance despite the capsule.
Glycosidic bond: A covalent bond linking a sugar's anomeric carbon to another group, often another sugar. The types and positions of these bonds help determine capsule structure and antigenicity.
Haemophilus influenzae type b vaccine: A vaccine that prevents invasive disease caused by Haemophilus influenzae serotype b. Its antigen is a capsule-derived polysaccharide linked to a carrier protein.
Neisseria meningitidis: A bacterial species that can cause meningitis and bloodstream infection. Its capsule helps the bacterium survive in blood and defines major vaccine-targeted serogroups.
O antigen: The variable polysaccharide portion of lipopolysaccharide on the outer membrane of many Gram-negative bacteria. Unlike a capsule, this antigen is attached to the cell's outer-membrane lipopolysaccharide.

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