Immune response to the radiotracer. Formation of high-molecular-weight immune complexes in plasma can be detected by HPLC as early as 3 to 4 weeks following incubation with the radiotracer. Radiochromatograms were transformed into probability density curves by normalizing for the area under the curve. Development of the immune response was dependent on TBI dose. (A) For monkeys who received 3Gyx2 TBI it is rapid, within 3 to 4 weeks or 7 months. (B) For 4Gyx2 TBI monkeys, the immune response is delayed, occurring approximately around 1 year post-TBI. (C) For 5Gyx2, an immune response to the radiotracer appears at 1.5 years. HPLC chromatograms are in parallel with the plasma-binding assay results (shown as an inset in each HPLC histogram). As a negative control, plasma from an unexposed monkey (black dash) was run through HPLC on the same day in addition to gel-filtration HPLC standard mixture (as shown in first panel) containing 5 standard components with different molecular weights (MW) (bovine thyroglobulin [MW 670 000 Da; brown], bovine γ-globulin [MW 158 000 Da; green], chicken ovalbumin [MW 44 000 Da; blue], horse myoglobin [MW 17 000 Da; purple], and vitamin B12 [MW 1350 Da; black]).