Anti-cardiolipin antibody (Anti-CL Ab) is one of the various anti-phospholipid antibodies (Anti-PL Abs) and found in the plasma of patients with systemic lupus erythematosus (SLE), atherosclerosis, and other infectious diseases. While anti-PL Abs found in the sera of patients with infectious diseases bind directly to CL, binding of anti-PL Abs to CL circulating in the sera of patients with autoimmune diseases is mediated by β₂-glycoprotein 1 (β₂-GP1). The purpose of this study is to investigate the effect of β₂-GP1 on the antigen binding assay of anti-CL Abs present in the sera of patients with atherosclerosis, which has been known as one of autoimmune diseases.

ELISA was performed with sera containing anti-CL Abs from three patients with atherosclerosis in the presence or absence of β₂-GP1 or FBS.

Reactivity of anti-CL Abs to CL was increased in the presence of β₂-GP1 or FBS in a dose dependent manner.

β₂-GP1 or FBS could be used as co-factor in CL ELISA with anti-CL Abs present in the sera of patients with atherosclerosis. It is suggested that anti-CL Abs found in atherosclerosis patients are similar in terms of antigen binding property to those circulating in the patients with autoimmune diseases, not to infectious diseases.