Which protein is NOT associated with the regulation of the complement system?

Factor D is not directly involved in the regulation of the complement system; rather, it plays a role in the activation aspect of the complement cascade. Specifically, Factor D is a serine protease that is essential for the activation of the alternative pathway of complement activation. It acts by cleaving Factor B when it is bound to C3b, thereby facilitating the formation of the C3 convertase, which is crucial for complement activation.

In contrast, the other proteins mentioned are all key regulators in the complement system. Factor H inhibits the alternative pathway by binding to C3b and promoting its degradation. DAF (decay-accelerating factor) hastens the decay of C3 and C5 convertases, thereby preventing overactivation of the complement system. CR1 (complement receptor 1) not only helps clear immune complexes but also functions to inhibit complement activation by facilitating the decay of convertases. Thus, Factor D, while essential for complement activation, does not play a regulatory role and is why it is identified as the protein that is not associated with the regulation of the complement system.