Changes in the titers of the antibody in the patient’s serum over the course of the disease. Normal (solid bar) and patient (open bars) serum samples were diluted (0.25 μL of serum in 100 μL of 2% BSA in PBS) and incubated in wells coated with 0.4 μg of thrombin-modified factor XIII (A₂′B₂). After overnight incubation at room temperature, the human IgG bound to the antigen was detected with an alkaline phosphatase-conjugated antibody (γ-chain specific). For experimental details, see Materials and Methods. Values shown are averages for duplicate samples. Inset: Normal (lanes 1 and 2) and patient plasmas (lanes 3 and 4; drawn 5/03/97) were treated with thrombin in the presence (lanes 2 and 4) and absence (lanes 1 and 3) of Ca²⁺ and the washed clots were analyzed by SDS-PAGE as in Fig 1A.