CD46 also known as MCP is a type I membrane protein and is a regulatory part of the complement system. CD46 plays a crucial role in regulating the complement system, which is essential for immune response and tissue homeostasis. CD46 functions by binding to complement components C3b and C4b, facilitating their degradation by serine protease factor I, thereby preventing damage to host tissues during immune responses. This regulatory mechanism is vital for maintaining self-tolerance and preventing autoimmune reactions. CD46 is expressed on a wide range of human cells, including thymocytes, T cells, B cells, monocytes, granulocytes, natural killer cells, platelets, endothelial cells, epithelial cells, fibroblasts, placenta, and sperm, but is absent on erythrocytes.