What is the role of decay-accelerating factor (DAF) and membrane co-factor proteins (MCP)?

Decay-accelerating factor (DAF) and membrane co-factor proteins (MCP) play critical roles in regulating the complement system, specifically in protecting human cells from unintended damage caused by complement activation. The complement system is a part of the innate immune response that enhances the ability to clear pathogens, but its activation must be tightly controlled to avoid harm to the host’s own tissues.

DAF helps to prevent the formation of the C3 convertase enzyme complex on the surface of human cells, which is essential for complement activation. By disrupting this convertase complex, DAF limits the complement cascade that could otherwise lead to cell lysis or inflammation. Similarly, MCP serves as a co-factor for the proteolytic enzyme factor I, which degrades C3b and C4b, thus inhibiting further complement activation on human cells.

The correct answer highlights that the primary function of DAF and MCP is to ensure that minimal complement is fixed to human cell surfaces, allowing the immune system to maintain the balance necessary for self-tolerance while still being able to respond effectively to pathogens. This mechanism is crucial for preventing autoimmune reactions where the body would attack its own cells, highlighting the importance of these regulatory proteins in immune homeostasis.