Fig. 3

Approaches for quantification of anti-SARS-CoV-2 immunoglobulins. A The setup of common serology tests using indirect ELISA allows for quantification of total IgG immunoglobulins, with no differentiation of distinct subclasses. Alternative assays by IP-SRM B–D facilitate comprehensive investigation of serological response. B IP-SRM assays allow for the direct, multiplex and differential quantification of immunoglobulin isotypes (IgG, IgM, IgA) and subclasses (IgG1-4, and IgA1-2) binding to protein antigens or linear epitopes coated onto microwell plates. “Absolute” quantification (pmol/mL or ng/mL) allows for the inter-laboratory standardization of serology assays using stable isotope-labeled peptide internal standards, and for investigation of cross-reactivity of the indirect ELISA. C Indirect IP-SRM assays provide additional tools for investigation of cross-reactivity of serology tests. D Surrogate neutralization assays by IP-SRM allow for quantification of immunoglobulins disrupting ACE2-spike glycoprotein interactions, and investigation of the impact of mutations emerging within the neutralizing linear epitopes