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Postgrad Med J 77:562-569 doi:10.1136/pmj.77.911.562
  • Review

Clinical implications of the specialised B cell response to polysaccharide encapsulated pathogens

  1. C G Vinuesaa,
  2. C de Lucasb,
  3. M C Cookc
  1. aMRC Centre for Immune Regulation, University of Birmingham Medical School, Edgbaston, Birmingham, UK, bDepartment of Paediatric Nephrology, Hospital San Rafael, Madrid, Spain, cCanberra Clinical School, University of Sydney and The Canberra Hospital, Woden, ACT, Australia
  1. Dr Matthew Cook, Canberra Clinical School, University of Sydney, PO Box 11, Woden, ACT, Australia, 2606Matthew.Cook{at}act.gov.au
  • Received 23 January 2001
  • Accepted 6 March 2001

Polysaccharide encapsulated human pathogens

Box 1: Summary of main points

  • Encapsulated bacteria (meningococci, pneumococci, and H influenzae type b) are major causes of respiratory and meningeal infections in infancy worldwide.

  • Factors that predispose to recurrent infection with encapsulated organisms reflect the importance of B cell receptor signalling and production of complement fixing, opsonising IgG antibodies for host defence against these pathogens.

  • Investigation of patients with recurrent infection with encapsulated bacteria hinges on identification of defects in antibody production (specific B cell defects, hyposplenism) and opsonisation defects (complement deficiency).

  • Capsular polysaccharide antigens evoke type 2 thymus independent (TI-2) antibody responses but fail to generate conventional B cell memory.

  • Antibody unresponsiveness to TI-2 antigens can be overcome by inducing T cell help for responses to polysaccharide antigens (for example, with conjugate vaccines).

include meningococci, pneumococci, and Haemophilus influenzae type b. These bacteria have caused substantial morbidity and mortality in humans since antiquity and are still the third leading cause of death in the world today.1 They account for the majority of childhood mortality from lower respiratory tract infections in developing countries, and are responsible for most cases of bacterial meningitis worldwide. Although these bacteria are members of distinct genuses, clinically they share a predilection for causing invasive disease, especially in young children. This review discusses how the clinical features common to these infections, including the shared predispositions, reflect the special immune response to polysaccharides, which occur in encapsulated bacteria (box 1). In particular, production of antibodies is critical for host defence against these organisms. A description of the basic immune response is used to explain the rationale of diagnostic and preventive strategies for patients with increased susceptibility to infections with encapsulated organisms.

Microbiology of encapsulated bacteria

Polysaccharide capsules confer virulence, in part because they enable bacteria to evade adaptive and specific immune defence mechanisms.2 …