Many bacteria, including Neisseria meningitidis (meningococcus), Streptococcus pneumoniae (pneumococcus), and H. influenzae, have an outer capsule composed of polysaccharides that are speciesand type-specific for particular strains of the bacterium. The most effective defense against these microorganisms is opsonization of the polysaccharide coat with antibody. The aim of vaccination for these organisms is therefore to elicit antibodies against the polysaccharide capsules of the bacteria. However, effective acellular vaccines cannot be made from a single isolated constituent of a microorganism, since generation of an effective antibody response requires the participation of several types of cells, and this fact has led to the development of conjugate vaccines (Fig. 16.27).
Capsular polysaccharides can be harvested from bacterial growth medium and, because they are T-cell-independent antigens (see Section 10-1), they can be used on their own as vaccines. However, young children under the age
Fig. 16.27 Conjugate vaccines take advantage of linked recognition to boost B-cell responses against polysaccharide antigens. The Hib vaccine against Haemophilus influenzae type b is a conjugate of bacterial polysaccharide and the tetanus toxoid protein. The B cell recognizes and binds the polysaccharide, internalizes and degrades the whole conjugate, and then displays toxoid-derived peptides on surface MHC class II molecules. Helper T cells generated in response to earlier vaccination against the toxoid recognize the complex on the B-cell surface and activate the B cell to produce anti-polysaccharide antibody. This antibody can then protect against infection with H. influenzae type b.
Chapter 16: Manipulation of the Immune Response
Fig. 16.28 The effect of vaccination against group C Neisseria meningitidis (meningococcus) on the number of cases of group B and group C meningococcal disease in England and Wales. Meningococcal infection affects roughly 5 in 100,000 people a year in the UK, with groups B and C meningococci accounting for almost all the cases. Before the introduction of the meningitis C vaccine, group C disease was the second most common cause of meningococcal disease, accounting for about 40% of cases. Group C disease now accounts for less than 10% of cases, with group B disease accounting for more than 80% of cases. After the introduction of the vaccine, there was a significant decrease in the number of laboratory-confirmed cases of group C disease in all age groups. The impact was greatest in the immunized groups, with reductions of more than 90% in these groups. An impact has also been seen in the unimmunized age groups, with a reduction of about 70%, suggesting that this vaccine has had a herd immunity effect.
Number of cases