  The size of a probe bead reported by holographic particle characterization
depends on the proportion of the surface area covered by bound target molecules
and so can be used as an assay for molecular binding. We validate this
technique by measuring the kinetics of irreversible binding for the antibodies
immunoglobulin G (IgG) and immunoglobulin M (IgM) as they attach to
micrometer-diameter colloidal beads coated with protein A. These measurements
yield the antibodies' binding rates and can be inverted to obtain the
concentration of antibodies in solution. Holographic molecular binding assays
therefore can be used to perform fast quantitative immunoassays that are
complementary to conventional serological tests.
