Final answer:
Factor I is the serine protease that cleaves and inactivates C3b and C4b with the help of cofactors such as MCP (CD46) or DAF (CD55).
Step-by-step explanation:
The serine protease that acts with cofactors to cleave and inactivate C3b and C4b is Factor I. Factor I requires the presence of cofactors such as Membrane Cofactor Protein (MCP, also known as CD46) or Decay-Accelerating Factor (DAF, also known as CD55), to function effectively. When Factor I is bound to a cofactor, it can cleave C3b and C4b, leading to the inactivation of these key components of the complement system and thereby regulating the immune response.