MBL (mannose-binding lectin) is a pattern recognition molecule and a component of innate immunity, the first line of the host defence system against foreign bodies and pathogens. MBL deficiency is common in humans and has been associated with immunodeficiency. We have generated mouse models of MBL deficiency in order to explore the molecular mechanisms of MBL function in disease and health. In addition to confirming findings from human clinical research, these model studies have uncovered unexpected roles of MBL and evidence of its interaction with other molecules of the innate immune system.