The Quellung (capsule swelling) reaction, used for serotyping encapsulated bacteria such as Streptococcus pneumoniae, is based on which principle?

• A Antibody activates complement leading to opsonization and actual capsule depolymerization
• B The capsule stain (India ink) is displaced by antibody binding, causing apparent capsule enlargement
• C Anticapsular IgM causes cross-linking of capsular chains, increasing their visible thickness on electron microscopy
• D Type-specific antibody binds to the polysaccharide capsule and the resulting antibody–antigen complex has greater light refractility, making the capsule appear swollen and more distinct under the microscope ✓

Explanation

The Neufeld Quellung reaction (German: Quellung = swelling) is an optical phenomenon. When type-specific anticapsular antibody (e.g., Omniserum or specific antisera) is mixed with a capsulated organism and methylene blue under a light microscope, the antibody-antigen complex on the capsule surface changes its refractive index relative to the surrounding medium, causing the capsule to appear sharply outlined, refractile, and 'swollen.' This allows serotyping of pneumococci (>90 serotypes) and other encapsulated pathogens (H. influenzae, K. pneumoniae).

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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