The activity of lupus anticoagulant and anticardiolipin antibodies appears to be directed against

• A. Factor V
• B. Factor VIII
• C. Factor IX
• D. Phospholipid

Answer: (D)

Lupus anticoagulant and anticardiolipin antibodies are antiphospholipid antibodies. Their activity is directed at phospholipid-containing surfaces that are essential cofactors in coagulation reactions. Lupus anticoagulant binds phospholipid-protein complexes on platelet and reagent surfaces, disrupting the assembly of coagulation enzyme complexes and prolonging phospholipid-dependent clotting tests. Anticardiolipin antibodies specifically recognize cardiolipin, a phospholipid, usually in complex with beta-2 glycoprotein I, and they interfere with phospholipid-dependent assays as well. Because these antibodies target phospholipid-containing structures rather than coagulation proteins like Factor V, VIII, or IX, their effects show up as prolonged clotting times in vitro and positive phospholipid-dependent immunoassays.