Mannose/mannan-binding lectin (MBL) is a secreted glycoprotein in the collectin family of pattern-recognition molecules. It contains a cysteine-rich region, a collagen-like segment, and a C-type lectin domain that binds to neutral bacterial carbohydrates. The collagen-like region mediates MBL association into disulfide-stabilized trimers which further associate into complexes containing three or four copies of the basic trimer. MBL is secreted by hepatocytes and opsonizes bacteria through interactions with microbial carbohydrates.
MBL multimers can associate with the serine proteases MASP-1, -2, and -3 and promote their cleavage of Complement Component C3. Proteolytic cleavage of C3 triggers activation of the complement system and formation of the membrane attack complex, leading to the destruction of opsonized bacteria. Opsonized apoptotic material is cleared by the scavenger receptor CD91.