Polysaccharide capsule of Haemophilus influenzae type b (PRP capsule (for "polyribosyl ribitol phosphate" capsule))

The **polysaccharide capsule of Haemophilus influenzae type b** is a surface polymer composed of repeating units of **polyribosyl ribitol phosphate (PRP)**, forming a thick, hydrophilic layer that envelops the bacterium[4][6][8]. This capsule is the predominant **virulence determinant** of Hib, providing **resistance to host immune mechanisms**—particularly **complement-mediated killing and phagocytosis**—by masking surface antigens and preventing immune recognition[1][4][6]. The capsule’s structure is a linear polymer with (→3)-β-D-Ribofuranosyl-(1→1)-D-ribitol-5-phosphate repeating units[9]. Genes involved in its biosynthesis are organized in a dedicated genetic locus with both polymerization and export machinery, and its presence is essential for invasiveness in humans[1][2]. The capsule is the target of **licensed Hib conjugate vaccines**, which couple size-reduced or native PRP to carrier proteins (such as tetanus toxoid) to elicit a T-dependent immune response and induce high-affinity, protective anti-capsular antibodies[7][9]. These antibodies neutralize the pathogen by promoting phagocytosis and complement fixation, and their serum levels serve as **biomarkers for vaccine-induced protection**[8]. The minimal immunogenic epitope consists of two repeating units of PRP, providing the basis for next-generation synthetic or enzymatic vaccine design[3][5][7]. The capsule is not a therapeutic target for small-molecule drugs; its main relevance is as a **vaccine antigen** and marker of infection. Safety concerns related to targeting the capsule are minimal and limited mainly to vaccine formulation components, not the capsule itself.

Other names

Haemophilus influenzae type b capsuleHib capsulePolyribosyl ribitol phosphate capsulePRP capsule

Elicitation of protective anti-PRP antibody response (via conjugate vaccines) Neutralization/opsonization by antibodies targeting the capsule, leading to bacterial clearance Prevents colonization and invasive infection