Three types of Experimental Animal Models have been used to investigate APA. 1) Passive transfer of APA to nocive mice. Branch et al injected pregnant Balb/c mice with purified IgG APA from patients with recurrent fetal loss. 2) Active immunization with APA. Backimer et al immunized Balb/c mice with a human monoclonal APA carrying the pathogenic idiotypes. 3) Immunization with β2 glycoprotein I (β2 GPI) (APA cofactor).Using affinity chromatography, we separated two distinct populations of antibodies. One was specific for β2GPI without binding to PL. Another population had a dual specificity for PL and β2