The overlap of CAPS and thrombotic antiphospholipid syndrome. The presence of aPLs and complement gene abnormality fosters a procoagulant and proinflammatory environment, which increases the risk for macrovascular (venous and arterial) thrombotic events in APS. Approximately 1% of patients with APS develop a severe picture of catastrophic APS (CAPS). The “2-hit” theory suggests that the presence of additional precipitating factors (including infection, inflammation, pregnancy, surgery, trauma, etc) leads to a domino effect that results in activation of the complement system, endothelium, and coagulation cascade, as well as immune cell activation, which are key contributors to the pathogenesis of CAPS.