Which of the following disrupts C3bBb formation on human cells?

The disruption of C3bBb formation on human cells is crucial because C3bBb is a key component of the alternative pathway of complement activation, which plays an important role in immune responses. To protect human cells from inappropriate complement activation, certain regulatory proteins intervene to inhibit the formation of this complex.

Both membrane cofactor protein (MCP) and decay-accelerating factor (DAF) serve as regulatory proteins that help to control the complement system. MCP works by serving as a cofactor for factor I, which cleaves C3b, thus preventing the stabilization of the C3bBb complex. This reduces the likelihood of complement-mediated damage to host cells. DAF, on the other hand, accelerates the decay of the C3bBb complex, effectively preventing it from persisting on the surface of human cells and thereby avoiding complement activation that could lead to cell lysis.

The combination of these functions provides a robust mechanism to protect human cells from overactive complement pathways, explaining why both MCP and DAF disrupt the formation of C3bBb on human cells. As a result, the choice that includes both B and C reflects the dual regulatory approach that these proteins take to maintain immune balance and protect against complement-mediated damage.