Apolipoprotein H (ApoH) is a 50 kDa variably glycosylated member of the complement control superfamily of proteins. Human ApoH is a major phospholipid binding protein and an important component to measure in the assessment of anti-phospholipid syndrome. Hepatocyte-derived ApoH binds to negatively charged phospholipids . It circulates as a component of lipoprotein particles and as a lipid-free serum protein. Human ApoH is also more specific than anti-cardiolipin antibodies and its presence correlates better with thrombotic risk. Mature human ApoH shares 76% and 82% aa sequence identity with mouse and rat ApoH.