kantigen

K antigen refers to a capsular polysaccharide antigen on the surface of certain bacteria, especially Klebsiella pneumoniae and Escherichia coli. It is a key component of the capsule and is used to serotype strains in clinical microbiology. The term, often written as K antigen or K capsule, distinguishes capsule type from other surface antigens such as O (somatic) and H (flagellar) antigens.

Structure and genetics: The capsule is a high-molecular-weight polysaccharide; capsule biosynthesis genes are located at the cps locus. Different serotypes correspond to distinct capsule structures, denoted by K1, K2, K4, etc. In Klebsiella, K1 and K2 serotypes are frequently linked to invasive disease; in E. coli, several K antigens (e.g., K1, K5) characterize uropathogenic and extraintestinal strains.

Function and clinical relevance: The capsule shields bacteria from host defenses, reducing phagocytosis and complement-mediated lysis, and can promote biofilm formation. K antigen typing informs epidemiology and outbreak investigations and can guide vaccine design in some contexts; detection is via serological methods such as the quellung reaction or slide agglutination, and increasingly by molecular methods targeting cps genes or whole-genome sequencing.

Notes: The term K antigen in this microbiological sense is distinct from the K antigen of the Kell blood group system, which is a different antigenic system entirely. Capsule diversity is a major factor in virulence and immune evasion across species that express K antigens.