Humoral pattern recognition molecules: mannan-binding lectin and ficolins

Innate immunity comprises a sophisticated network of molecules, which recognize pathogens, and effector molecules, working together to establish a quick and efficient immune response to infectious agents. Complement activation triggered by mannan binding lectin (MBL) or ficolins represents a beautiful example of this network Both MBL and ficolins recognize specific chemical structures on the surface of antigens and pathogens, thus bind to a broad variety of pathogens. Once bound further complement deposition is achieved through a cascade of proteolytic reactions. MBL and ficolin induced complement activation is critical for adequate anti-bacterial, anti-fungal and anti-viral responses. This is well illustrated by numerous and convincing studies that demonstrate associations between MBL deficiency and infections. Recent work has also highlighted that MBL and ficolins recognize self-structures, thus extending the role of these molecules beyond the traditional view of first line defense molecules. It appears that MBL deficiency may modulate the prognosis of inflammatory and autoimmune diseases. What is known about the mechanisms behind this broad scope of activities of MBL and ficolins is discussed in this chapter.