Which bacterial structural component often serves as the antigenic determinant for serotyping and helps evade host immune responses?
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Correct answer: Capsular polysaccharide
The capsular polysaccharide is the classic answer: capsular (K) antigens define many serotypes (e.g., the 90+ pneumococcal capsular serotypes and the K antigens of E. coli, H. influenzae, and N. meningitidis), and the capsule is the principal immune-evasion structure because it resists phagocytosis and complement. Flagella (H antigens) are also used for serotyping, but the determinant is flagellin in the filament—not the hook—and flagellin is strongly immunogenic (recognized by TLR5) rather than immune-evading. Lipoteichoic acid and outer-membrane porins are not classic serotyping/immune-evasion structures.
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