Antibody repertoires in infants and adults: effects of T-independent and T-dependent immunizations.

Polysaccharide(PS)-encapsulated bacteria such as Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis are among the most prevalent bacterial pathogens of humans. Infections caused by these organisms are both common (otitis media, sinusitis) and severe (meningitis, bacteremia). Antibodies directed against the capsular PS of encapsulated bacteria prevent infection by promoting opsonophagocytic killing. Most bacterial PS, however, are type II T-cell-independent (TI-2) antigens that are poorly immunogenic in young children at highest risk of developing disease. Conjugation of bacterial PS to a protein carrier converts the immune response to a T-cell-dependent (TD) form and significantly improves the immunogenicity of PS, especially in infants. H. influenzae type b (Hib) is a major cause of invasive infection in non-immune children. The medical importance of this pathogen and the availability of both TI-2 and TD Hib PS vaccine formulations have made the human anti-Hib-PS immune response an excellent model for the study of the biology of these B cell responses.