Activation of the lectin pathway begins when mannan-binding protein (MBP) binds to the mannose groups of the microbial carbohydrates. Two more lectin pathway proteins called MASP1 and MASP2 (equivalent to C1r and C1s of the classical pathway) now bind to the MBP. This forms an enzyme similar to C1 of the classical complement pathway that is able to cleave C4 and C2 to form C4bC2a, the C3 convertase capable of enzymatically splitting hundreds of molecules of C3 into C3a and C3b.