The Quellung reaction is a biochemical reaction in which antibodies bind to the bacterial capsule of Streptococcus pneumoniae, Klebsiella pneumoniae, Neisseria meningitidis, Haemophilus influenzae, Escherichia coli, and Salmonella. The antibody reaction allows these species to be visualized under a microscope. If the reaction is positive, the capsule becomes opaque and appears to enlarge.
Quellung is the German word for "swelling" and describes the microscopic appearance of pneumococcal or other bacterial capsules after their polysaccharide antigen has combined with a specific antibody. The antibody usually comes from a bit of serum taken from an immunized laboratory animal. As a result of this combination, and precipitation of the large, complex molecule formed, the capsule appears to swell, because of increased surface tension, and its outlines become clearly demarcated.
The pneumococcal Quellung reaction was first described in 1902  by the scientist Fred Neufeld, both as microscopic capsular swelling and macroscopic agglutination (clumping visible with the naked eye). It has been used to identify the 93 known capsular serotypes of Streptococcus pneumoniae in a diagnostic setting, but in recent years has been challenged by molecular typing techniques which target genetic differences.
- Fisher, Bruce; Harvey, Richard P.; Champe, Pamela C. Lippincott's Illustrated Reviews: Microbiology (Lippincott's Illustrated Reviews Series). Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 0-7817-8215-5. Page 340
- Neufeld, F (1902). "Ueber die Agglutination der Pneumokokken und uber die Theorien der Agglutination.". Zeitschrift fur Hygiene Infektionskrankheiten. pp. 54–72.
- PCR Deduction of Pneumococcal Serotypes. Center for Disease Control. Retrieved October 22, 2010.
- Park, I. H., Pritchard, D., Cartee, R., Brandao, A., Brandileone, M. & Nahm, M. (2007). Discovery of a new capsular serotype (6C) within serogroup 6 of Streptococcus pneumoniae. J Clin Microbiol 45, 1225–1233.
- Pai R, R. E. Gertz, B. Beall. 2006. Sequential multiplex PCR approach for determining capsular serotypes of Streptococcus pneumoniae isolates. J. Clin. Microbiol. 44:124-31.
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