Mannose Binding Lectin (MBL) is a C-type lectin that is an important element in innate immunity. MBL belongs to the collectin family of proteins that consists of a collagen-like domain and a carbohydrate recognition domain (CRD). Through this, MBL recognizes carbohydrates (e.g. mannose and N-acetylglucosamine) on pathogens. MBL forms several sizes of oligomers that are composed of subunits of three identical 32 kDa polypeptides. MBL is a pattern recognition receptor, and upon recognition of the infectious agent, MBL triggers the activation of the lectin-complement pathway through attached C1r/C1s-like serine proteases, designated MASP. The MBL-MASP complex proteolytically cleaves C4, C2 and C3. The lectin pathway is antibody and C1q-independent and is, therefore, independent of the classical and alternative complement activation pathway. MBL is synthesized by hepatocytes and has been isolated from the liver or serum of several vertebrate species.Normal human plasma contains MBL concentrations ranging from 10 to 5,000 ng/ml. Up to 12% of healthy Caucasian blood donors have MBL concentrations below 100 ng/ml. Low plasma concentrations have been associated with an inherited defect in opsonization. The MBL concentration is enhanced in infectious diseases. Measuring of MBL is indicated in recurrent infections (especially in children), primary/secondary immunodeficiencies, artherosclerosis/coronary heart disease, cystic fibrosis, transplantation, autoimmune diseases (SLE/Rheumatoid arthritis) and habitual abortion. MBL measurement is not affected by the presence of antibodies against mannan. In the human MBL assay, any influence of the classical pathway has been eliminated by using a special MBL-binding buffer, which inhibits the binding of C1q to immunocomplexes and disrupts the C1 complex, while leaving the function of the MBL complex intact.