Trypsin-like serine protease that plays an essential role in regulating the immune response by controlling all complement pathways. Inhibits these pathways by cleaving three peptide bonds in the alpha-chain of C3b and two bonds in the alpha-chain of C4b thereby inactivating these proteins. Essential cofactors for these reactions include factor H and C4BP in the fluid phase and membrane cofactor protein/CD46 and CR1 on cell surfaces. The presence of these cofactors on healthy cells allows degradation of deposited C3b by CFI in order to prevent undesired complement activation, while in apoptotic cells or microbes, the absence of such cofactors leads to C3b-mediated complement activation and subsequent opsonization. Inactivates complement subcomponents C3b, iC3b and C4b by proteolytic cleavage. Heterodimer of a light and heavy chains; disulfide-linked. The fully processed and mature protein circulates as a zymogen, and is allosterically activated by substrate-induced remodeling of the active site. Interacts with C3b. Interacts with complement factor H. Expressed in the liver by hepatocytes. Also present in other cells such as monocytes, fibroblasts or keratinocytes. Belongs to the peptidase S1 family.