Summary of the pathogenesis of TMAs. TTP is the result of a severe deficiency of ADAMTS13, due to an inherited genetic effect in congenital TTP or due to antibodies against ADAMTS13. The result is excess platelet binding to ultra large von Willebrand factor (UL VWF) and thrombi formation, made of these constituents. Triggering of complement (CM-HUS), due to abnormalities in CFH, CFI, MCP, C3, or Factor B, results in overactivation of the terminal complement pathway, the membrane attack complex (MAC), resulting in endothelial damage and thrombi composed of platelets. IA-HUS and resulting toxin, precipitants resulting in DIC and TMAs not associated with severe ADAMTS13 deficiency, cause direct endothelial damage. Resultant activation of complement, release of VWF (including high ^+/− some UL VWF multimers), causes a reduction in ADAMTS13 levels because of consumption of the enzyme. Thrombi formation is distinct from TTP and CM-HUS, containing a variable proportion of platelets, inflammatory cells, or fibrin. Resulting thrombi cause MAHAT seen clinically.