Hemolytic uremic syndrome is a triad of microangiopathic hemolytic anemia, 
thrombocytopenia, and acute renal failure. It is one of a group of conditions 
termed the thrombotic microangiopathies, which are characterized by prominent 
endothelial cell injury. It may be diarrheal-associated or atypical (aHUS). 
Evidence for a pathogenic role of the alternative pathway of complement was 
first suggested in 1974. Mutations in the complement regulatory proteins factor 
H, membrane cofactor protein (CD46), and factor I predispose to aHUS 
development. Mutations of the activating components factor B and complement C3 
have also been reported. Penetrance is approximately 50%, suggesting other 
genetic and environmental modifiers are needed for disease expression. 
Identification of mutations is important owing to differences in mortality, 
renal survival, and outcome of renal transplantation. Current treatment is 
plasma infusion/exchange, but complement inhibitor therapy provides hope for the 
future.