Objective. To investigate the association of β2- glycoprotein I (β2GPI) with lipid rafts in monocytic cells and to evaluate the proinflammatory and procoagulant effects of anti- β2GPI binding to its target antigen on the monocyte plasma membrane. Methods. Human monocytes were fractionated by sucrose density-gradient centrifugation and analyzed by Western blotting. Immunoprecipitation experiments were performed to analyze the association of β2GPI with lipid rafts and the possible interaction of β2GPI with annexin A2 and Toll-like receptor 4 (TLR-4). Monocytes were then stimulated with affinity-purified anti-β2GPI antibodies from patients with the antiphospholipid syndrome (APS). Interleukin-1 receptor-associated kinase (IRAK) phosphorylation and NF-κB activation were evaluated by immunoprecipitation and transcription factor assay, respectively. Supernatants from monocytes were tested for tumor necrosis factor α (TNFα) and tissue factor (TF) levels by enzyme-linked immunosorbent assay. Results. We found β2GPI and its putative receptor annexin A2 in lipid raft fractions of human monocytes. Moreover, there was an association between β2GPI and TLR-4, suggesting that it was partially dependent on raft integrity. Triggering with anti-β2GPI antibodies induced IRAK phosphorylation and consequent NF-κB activation, which led to the release of TNFα and TF. Conclusion. Anti-β2GPI antibodies react with their target antigen, likely in association with annexin A2 and TLR-4, in lipid rafts in the monocyte plasma membrane. Anti- β2GPI binding triggers IRAK phosphorylation and NF-κB translocation, leading to a proinflammatory and procoagulant monocyte phenotype characterized by the release of TNFα and TF, respectively. These findings provide new insight into the pathogenesis of APS, improving our knowledge of valuable therapeutic targets. © 2007, American College of Rheumatology.