Membrane cofactor proteins (MCP) primarily function by:

Membrane cofactor protein (MCP), also known as CD46, plays a crucial role in regulating the complement system, specifically by aiding in the protection of host cells from complement-mediated damage. MCP primarily functions by acting as a cofactor for the enzyme factor I, which cleaves C3b when it is bound to the membrane. This action effectively inhibits the binding of C3b to human cells, thereby preventing the formation of the C3 convertase and subsequent complement activation that could lead to cell lysis or damage.

As a consequence of this mechanism, MCP helps to maintain self-tolerance and protect tissues from inappropriate immune responses. This is important in avoiding potential autoimmune reactions where the body could attack its own cells. The role of MCP in inhibiting C3b binding is fundamental in distinguishing self from non-self within the immune response.

In contrast, other options such as promoting inflammation, facilitating C3b deposition, or activating T cells do not accurately represent the primary function of MCP. Rather, they pertain to different aspects or pathways of immune response that do not directly involve the functions of membrane cofactor proteins.