The cell wall of H. influenzae is typical of other Gram-negative bacilli; lipopolysaccharide with endotoxin activities is present in the cell wall. Species-specific and strain-specific proteins are present in the outer membranes. Three major surface anti-gens are present in H. influenzae. They are (a) capsular poly-saccharide antigen, (b) outer membrane protein (OMP), and (c) lipooligosaccharide.
Capsular polysaccharide antigen is the major antigenic deter-minant of encapsulated H. influenzae. This antigen is present in the capsulated strain, which produces a capsule that is polysac-charide in nature. This antigen is not found in unencapsulated strains of H. influenzae. The polysaccharide antigen confers type specificity to the organism and is the basis for grouping of organism into six antigenic serotypes. These six antigenic sero-types are designated as a, b, c, d, e, and f.
Type b capsular polysaccharide is a host-protective antigen, which contains polyribosyl ribitol phosphate (PRP), hence is used in a number of PRP vaccines and PRP–protein conjugate vaccines. These proteins induce IgG, IgM, and IgA antibodies which are bactericidal, opsonic, and protective. H. influenzae serotype b was responsible for more than 95% of all invasive Haemophilusinfection before the introduction of H. influenzae type b (Hib) vaccines. But after the introduction of Hib vac-cine, specifically directed against H. influenzae serotype b, most disease caused by this serotype has been reduced. After the vaccination era, serotypes c and f as well as nonencapsulated H. influenzae are responsible for most H. influenzae disease.
The OMP antigens show considerable variations. The OMP antigens of Hib have been classified into 13 subtypes.
Lipooligosaccharide is antigenically complex.