Abstract
This study introduces a holographic agglutination assay for quantifying levels of the immunoglobulin protein IgA in biological samples. This is the first example of a label-free and bead-free assay that quantifies protein agglutinates by direct detection using Total Holographic Characterization. A proof-of-concept assay for human serum immunoglobulins is demonstrated using Jacalin, the galactose-specific plant lectin, to induce selective agglutination. By analyzing the size, refractive index, and number of particles in an assay sample, we obtain a reproducible and quantitative measurement of galactosylated immunoglobulins in a given sample. The assay is calibrated for a reference interval of IgA concentrations from 4 mg dL(-1) (0.25 μM) to 32 mg dL(-1) (2 μM) with a limit of detection (LOD) of 6.3 mg dL(-1) (0.39 μM) in emulated biological samples. This covers the physiologically relevant concentration range with ten-fold dilution. The assay clearly distinguishes samples containing IgA from samples containing IgG. More broadly, this study introduces a platform for creating lectin-mediated holographic agglutination assays to monitor levels of immunoglobulins in biological samples. The ability to quantify immunoglobulin levels efficiently in clinical samples is likely to be valuable for diagnostics and will provide a basis for assaying other proteins that can be induced to agglutinate.