MASP is a component of the bactericidal ra-reactive factor rarf which specifically binds to RA and R2 polysaccharides expressed by certain enterobacteria. It triggers the activation of complement cascade by activating the C4 and C2 components. It activates the C4 component by cleaving the alpha-chain of C4. The alternative-splicing product from MASP1 gene is called MASP3. MASP1 is associated with smaller MBL oligomers whereas MASP3 is found on larger oligomers. The substrate of MASP3 is unknown.