Quellung reaction

The Quellung reaction is a biochemical reaction in which antibodies bind to the capsule of Streptococcus pneumoniae and thus allow it 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.