CD59, also known as membrane inhibitor of reactive lysis (MIRL) or protectin, is a glycosylphosphatidylinositol (GPI)-anchored membrane protein that plays a crucial role in regulating the complement system, an essential part of the innate immune response. It is widely expressed on various human cells and tissues, including erythrocytes, leukocytes, fibroblasts, and epithelial cells. The primary function of CD59 is to inhibit the formation of the membrane attack complex (MAC), thereby preventing complement-mediated cell lysis. This protective mechanism is vital for maintaining cell integrity and preventing excessive immune-mediated damage.