Hoy et al. sought to evaluate the presence, clinical associations, and potential mechanistic roles of circulating calprotectin in a cohort of patients with primary antiphospholipid syndrome (APS) and those with antiphospholipid antibodies. Calprotectin levels were higher in patients with primary APS and those with antiphospholipid antibodies than in healthy controls. These data suggest that calprotectin has the potential to be a functional biomarker and a new therapeutic target for APS-related thrombocytopenia.