Streptoccus pnuemoniae (S. pneumoniae) is an encapsulated
Gram-positive bacterium. The capsule is a mucus-like layer
of polysaccharide, which ‘coats’ the bacterium, enabling it to
evade the immune system by preventing phagocytosis (the
process where white blood cells, phagocytes, recognise and
destroy foreign particles, such as bacteria or viruses) (Murphy
et al, 2008). There are at least 90 variations of pneumococcal
polysaccharide capsule (serotypes), meaning that an antibody
would need to be produced to recognize each one for a
person to become immune. Thankfully, most infections
are caused by a comparatively small number of serotypes