Which polysaccharide capsule antigen is found in Haemophilus influenzae type b (Hib), the most virulent serotype?

Explanation

## Haemophilus influenzae Capsular Antigens **Key Point:** Haemophilus influenzae type b (Hib) possesses a polyribose phosphate (PRP) capsule, which is the primary virulence factor and the target of the Hib vaccine. ### Capsular Types Haemophilus influenzae is classified into 6 serotypes (a–f) based on capsular polysaccharide composition: | Serotype | Capsular Antigen | Clinical Significance | | --- | --- | --- | | Type a | Polyribose phosphate (PRP) | Rare; causes invasive disease | | **Type b** | **Polyribose phosphate (PRP)** | **Most common invasive serotype; vaccine target** | | Type c | Polyribose phosphate (PRP) | Rare | | Type d | Polyribose phosphate (PRP) | Rare | | Type e | Polyribose phosphate (PRP) | Rare | | Type f | Polyribose phosphate (PRP) | Rare | | Unencapsulated | None | Causes non-invasive disease (otitis, sinusitis) | **High-Yield:** All capsulated Haemophilus influenzae serotypes (a–f) contain PRP, but **type b** is the most virulent and was historically the leading cause of bacterial meningitis in children before Hib vaccination. ### Mechanism of Virulence The PRP capsule: 1. Inhibits complement deposition and opsonization 2. Resists phagocytosis by non-immune cells 3. Evades the innate immune response 4. Enables bloodstream invasion and CNS penetration **Clinical Pearl:** The Hib conjugate vaccine (PRP-T, PRP-OMP) conjugates the PRP capsule to a protein carrier (tetanus toxoid or meningococcal outer membrane protein) to generate T-cell-dependent immunity in infants, providing protection from age 2 months onward. **Mnemonic:** **PRP = Polyribose Phosphate = Pathogenic (all Hib serotypes)**