11.3 Polysaccharide vaccines
The very diversity of polysaccharides may diminish the possibility of using them as vaccines. There are frequently too many different chemotypes for it to be practicable to prepare a potential vaccine. However, several bacterial diseases are caused by a small number of polysaccharide-producing serotypes as the dominant causes of infection. It is then possible to evaluate such polymers to determine whether they might be suitable for vaccine use. The antibody response to polysaccharide antigens varies greatly and can be affected by a number of factors including both chemical and physical properties of the exopolysaccharides. The specific chemical structure of the polysaccharide clearly plays a major role. In addition, the conformation in solution and the molecular mass are likely to affect antigenicity. As the immune response to polysaccharide is T-cell independent, this accounts for the poor immunogenicity of most polysaccharides. There is a lack of memory response and in humans a late response occurs. Thus the population most at risk to several infections caused by capsulate bacteria, infants of less than two years. Shows a very poor immune response. Another potential problem is the possible role of genetic factors in regulating immune response to bacterial polysaccharides. This may affect the susceptibility of certain infants to infections caused by polysaccharide-producing bacteria.