CD59, also known as the membrane attack complex inhibition factor (MACIF), is a membrane-bound protein that plays a critical role in the regulation of the complement system, which is part of the immune system. It is the primary inhibitor of membrane attack complex (MAC) formation. It prevents the formation of pores in the membranes of cells, thereby protecting them from complement-mediated lysis. CD59 inhibits the formation of MAC by binding to C8 and/or C9, which are components of the complement system. This binding prevents the recruitment of additional C9 molecules into the complex, thus inhibiting the formation of the pore. CD59 deficiency can lead to conditions such as paroxysmal nocturnal hemoglobinuria (PNH), which is characterized by intravascular hemolysis and hemoglobinuria. Low levels of CD59 are also associated with autoimmune diseases like diabetes and multiple sclerosis.