The immune system of children under the age years of 5 does not respond well to polysaccharides.
Polysaccharides are long chains of sugars.
Many bacteria coat themselves with polysaccharide. The outer coating is called a capsule. Exactly why children do not respond to these bacterial capsules is not fully understood as yet. However, researchers have discovered that if a the bacterial capsular polysaccharide is bound to a protein the children are able to make antibodies to the capsule. When antibodies are made to the capsular material they can bind to the surface of the bacteria to aid the white blood cells in phagocytizing and destroying the bacteria.