Thrombotic microangiopathies (TMA) are a category of diseases linked by endothelial injury leading to aggregation of platelets on the damaged endothelium, microvascular thrombosis and organ dysfunction related to microvascular injury.  Disorders include Shiga toxin-producing E. coli-associated hemolytic uremic syndrome (STEC-HUS), atypical hemolytic uremic syndrome (aHUS) and thrombotic thrombocytopenic purpura (TTP).

Atypical HUS is associated with defects in the regulation of the alternative complement pathway leading to uninhibited formation of the C3 convertase C3bBb on the endothelium and subsequent microvascular injury, whereas TTP is caused by the inability of ADAMTS13 to cleave ultra-large multimers of von Willebrand Factor (vWF), with resulting adhesion of these ultra-large vWF multimers to the endothelial surface under shear stress leading to microvascular thrombosis and injury. 

In addition, many secondary forms of TMA including stem cell transplant- and solid organ transplant-associated HUS, HELLP syndrome, and others have recently been associated with defects in complement regulation.