Fig. 3

Serological diagnostics with immunoaffinity-targeted proteomics. (A) Specific antibodies are affinity enriched from the patient serum, denatured, and digested with trypsin, and the peptides are analyzed by targeted MS assays. (B) Following electrospray ionization (ESI), targeted MS assays, such as Selected Reaction Monitoring, enable the isolation of peptide ions of interest with the first quadrupole (Q1), their fragmentation in Q2, isolation of fragments of interest in Q3, and measurement of fragment ion intensities. (C) Peak areas of the “light” endogenous peptides and the corresponding spiked-in “heavy” isotope-labeled internal standards are used to calculate peptide ratios and absolute concentrations of the complete panel of the human immunoglobulin isotypes and subclasses