Of the several species of Haemophilus that are known, Haemophilus influenzae is the most prevalent pathogen. Several distinct capsular serotypes have been defined, but type b H influenzae is responsible for most clinical disease. H influenzae is a respiratory pathogen that colonizes the oropharynx and causes bronchitis, pneumonia, or disseminated infection when local or systemic host defense factors are compromised.
Haemophilus Influenzae type b capsule is a polyribitol phosphate. The susceptibility of a given host to infection is directly related to serum levels of bactericidal antibody. Maternal IgG is lost within a few months after birth, and natural antibody is acquired by 3-4 years of age. The development of natural antigen antibody may be related to colonization and subsequent immunization vaccines with nonpathogenic members of the family Enterobacteriaceae that contain cross-reactive capsular polysaccharides.
H influenzae type b capsular polysaccharide (polyriboseribitol phosphate, PRP) is a weak immunogen, and vaccines have not been effective in children younger than 2 years. However, conjugate vaccines with diphtheria toxoid, tetanus toxoid, and outer membrane proteins linked to PRP have proven effective in immunizing children between 6 months and 2 years of age. In few bacterial diseases has the protective role of antibody been so clearly demonstrated as in H influenzae type b disease. The widespread use of the new conjugate vaccines has nearly eradicated meningitis due to H influenzae in the United States.
