O2 polysaccharide antigen

The **O2 polysaccharide antigen** is a specific type of *O-antigen*—the variable polysaccharide portion of the lipopolysaccharide (LPS) layer on the outer membrane of certain Gram-negative bacteria, most notably *Klebsiella pneumoniae* serotype O2[1][3][6][8]. The O2 antigen is distinguished by a **D-galactan I backbone** composed of repeating disaccharide units: [→3)-α-D-Galp-(1→3)-β-D-Galf-(1→], produced by enzymes encoded primarily in the *rfb* locus[1][3]. This antigenic structure can be further modified by side chains or *O*-acetylation, resulting in multiple closely related serotypes such as O2a, O2aeh, O2afg, and O2ac, each with additional glycosyl links or substitutions that affect immune recognition[1][3]. The O2 antigen is a major surface-exposed immunogen, critical for resistance to host complement, influencing virulence, immune evasion, and vaccine target potential[1][2][7]. Antibodies against O2 antigen can mediate bacterial killing, and the polysaccharide is considered an attractive candidate for vaccine or passive immunization strategies given its immunodominance and restricted variability[1][4]. However, antigenic diversity and structural modifications pose challenges for therapeutic targeting or vaccine breadth[1][3][6].

Other names

O2 antigenO2a antigenD-galactan I (referring to the backbone structure)O-antigen (in the context of Klebsiella pneumoniae O2 serotype)O2aeh, O2afg, O2ac antigen (when referring to O2-antigen backbone with distinct modifications)

Antibody binding leading to complement activation or opsonization (bacterial killing) Preventing bacterial attachment and colonization through vaccine-induced or therapeutic antibody responses