The multifunctional role of antibodies in the protective response to bacterial T cell-independent antigens.

While most complex antigens can induce antibody responses in a mature immunological system, this is not the case when injected into ontogenetically immature systems, as are found in neonates and pediatric-age children. Thus the antibody response to polysaccharides, which would in theory provide protection against infection by all polysaccharide encapsulated bacteria, including Streptococcus pneumoniae, Neisseria meningitides, and Haemophilus influenzae, cannot be stimulated by immunization with the polysaccharides by themselves. It was only with the introduction of conjugate vaccines that protection from these bacterial infections was provided to this susceptible age group. The introduction of these conjugate vaccines into the arsenal of vaccines serves as a remarkable example of how valuable it is to understand the mechanisms of biological processes. Many years of intense laboratory investigation demonstrated that when polysaccharides are covalently conjugated to proteins, the characteristics of the immune response are similar to that of the protein rather than the polysaccharide. These characteristics would induce an anti-polysaccharide response even in the pediatric population, which was heretofore unable to mount protective responses to the polysaccharide. With the advent of conjugate vaccines for the above three mentioned bacteria, the incidence of bacteremia, meningitis, and otitis media has almost been eliminated. This chapter discusses in some detail the mechanisms which underlie the effectiveness of conjugate vaccines and discusses some of the vaccines that have been commercialized.