Immune responses to polysaccharides: lessons from humans and mice.

This review focuses on the immune response to non-conjugated and conjugated polysaccharide vaccines derived from encapsulated pathogens, such as Streptococcus pneumoniae, Haemophilus influenzae and Neisseria meningitidis. Special attention is paid to a number of side effects observed following the use of some of these vaccines. For example, we discuss the long-lasting specific refractoriness induced by unconjugated polysaccharides, and the absence of an effective immune response in adults vaccinated with some conjugated vaccines. We argue that studies performed in the mouse model can help to understand those paradoxical effects observed in humans, and the mechanisms underlying such processes.