Figure 2. Graph showing anti–protective antigen (PA) immunoglobulin G (IgG) titers plotted against 3H-thymidine counts per minute (cpm) derived from PA-stimulated blood cell cultures conducted in year 2. The vertical lines define the ELISA borderline and upper thresholds (10 IU/mL and 15 IU/mL, respectively), which were defined as PA titers by the ELISA kit manufacturer, i.e., titers supposed to confer protection after vaccination. Samples testing below the borderline threshold are considered negative. T29 and T2 refer to workers whose samples had a remarkably high antibody titer or lymphocyte proliferation count, respectively.