The immune system must discriminate between self and non-self. Mannan-binding lectin (MBL) is a recognition molecule able to differentiate between the carbohydrates found on self glycoproteins and the carbohydrate patterns found on infectious non-self surfaces. It exists in a complex with MBL-associated serine proteases (MASPs). When MBL binds to suitable carbohydrate pattern it causes activation of MASPs leading to triggering of the complement cascade. This results in limiting the infection and the orchestration of subsequent adaptive immune response. The plasma concentration of MBL is determined by genetic polymorphisms. Deficiency of MBL is a risk factor for infection, especially when other functions in the immune system are also compromised. MBL has a potential to react against altered-self structures, as found on apoptotic cells, cancers or ischemic-reperfused tissue. The focus of the current review is to summarize the recent progress in our understanding of MBL functions.