Educational resource · Not a medical device · Infectious diseases review in progress · Does not replace clinical judgement

Diagram — Polysaccharide capsule — SeptiMap

explain why the capsule shields bacteria from phagocytosis, and derive the risk of asplenia and the principle of conjugate vaccines. Educational resource, not a medical device.

Diagrams: mechanisms and principles

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Preclinical · clinical years

Polysaccharide capsule

Objective: explain why the capsule shields bacteria from phagocytosis, and derive the risk of asplenia and the principle of conjugate vaccines.

Step 5 / 5
Polysaccharide capsule1. The capsule — A polysaccharide layer surrounds the bacterium. Pneumococcus has more than 90 capsular serotypes. 2. Phagocytosis blocked — The capsule limits complement (C3b) deposition on its surface and prevents phagocytosis, whether mediated by antibody, complement or non-opsonic receptors. 3. Anticapsular antibodies — In vaccinated people, anticapsular antibodies are key to immunity against pneumococcus. 4. Conjugate vaccine — Polysaccharide linked to a carrier protein induces a T-dependent response that works in infants, unlike plain polysaccharide. 5. Asplenia — Asplenia and hyposplenism expose to overwhelming, high-mortality infections with encapsulated bacteria.CapsuleNo antibodyPhagocytePhagocytosisblockedWith antibodyAnticapsularImmunity ofthe vaccinatedConjugate vaccine: T-dependentresponse, from infancyAsplenia: overwhelming infectionwith encapsulated bacteria
  • Pathogen
  • Host (cells, tissues)
  • Immunity
  • Clinical consequence

Key points

  • The capsule limits complement deposition and prevents phagocytosis.
  • Conjugate vaccine: carrier protein, T-dependent response, effective in infants.
  • Asplenia: overwhelming infections with encapsulated bacteria.

Classic pitfall

Assuming a plain polysaccharide vaccine protects infants: without a carrier protein there is no T-dependent response.

Related pathogen pages (6)

Sources

Educational content · Illustrative mechanisms · Not applicable to individual clinical decisions · MDR 2017/745

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