Atypical hemolytic uremic syndrome

A rare thrombotic microangiopathy in which dysregulated complement activity damages small blood vessels, causing hemolytic anemia, thrombocytopenia, and often acute kidney injury.

Alternative complement pathway: A continuously active arm of innate immunity that amplifies complement activation on microbial surfaces. Its unchecked amplification is the central mechanism in many cases.
Microangiopathic hemolytic anemia: Anemia caused by red blood cells fragmenting as they pass through damaged small vessels. Fragmented cells on a blood smear are a hallmark of the syndrome.
Shiga toxin–producing Escherichia coli: A group of Escherichia coli strains that produce toxins capable of causing bloody diarrhea and hemolytic uremic syndrome. Its toxin-associated syndrome is the classic infectious counterpart to complement-mediated disease.
Complement inhibitor: A drug that suppresses one or more steps in the complement immune cascade. This drug class targets the pathway driving many cases of the syndrome.
Complement factor H autoantibody: An antibody directed against complement factor H that can impair its regulatory function. This acquired cause can produce the same complement-driven vascular injury as genetic variants.
Complement factor H: A soluble complement regulator that limits alternative-pathway activity on host cell surfaces. Variants can weaken protection of endothelial cells from complement attack.
Thrombocytopenia: A lower-than-normal platelet count in the blood. Platelets are consumed in small-vessel thrombi, often lowering their circulating count.
Thrombotic thrombocytopenic purpura: A thrombotic microangiopathy usually caused by severe deficiency of the enzyme ADAMTS13. Severe ADAMTS13 deficiency identifies a distinct emergency requiring different treatment.
Ravulizumab: A long-acting monoclonal antibody that inhibits complement component C5. It offers sustained terminal complement blockade for eligible patients.
Complement factor B: A complement protein that participates in forming the alternative pathway’s C3 convertase. Activating variants can increase pathway activity and predispose to disease.

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