This study introduces an antigen-specific liquid chromatography-mass spectrometry (LC-MS)-based IgG1 Fab profiling approach to analyze the molecular resolution of human autoantibody repertoires in rheumatoid arthritis (RA). The method uses the anti-citrullinated protein antibody (ACPA) repertoire as an example. The results show that each RA patient has a unique and diverse ACPA IgG1 repertoire dominated by a few antibody clones. In contrast to the total plasma IgG1 repertoire, the ACPA IgG1 sub-repertoire is characterized by an expansion of antibodies with one, two, or more Fab glycans, and different glycovariants of the same clone can be detected. The study indicates that the autoantibody response in RA is complex, unique to each patient, and dominated by a relatively low number of clones. Antibodies play a central role in protecting the host from pathogens, with each antibody harboring a highly variable region in its antigen-binding fragments (Fab). This variable domain is generated by recombination of variable (V), diversity (D), and joining (J) gene segments. The various processes may eventually give rise to billions of unique antibodies, including various clones capable of binding the same antigen. This high diversity and flexibility of the antibody repertoire allows antigen-specific immune responses even against newly arising or continuously evolving pathogens. To prevent the formation of antibodies that target the body itself, various tolerance mechanisms are in place. These mechanisms identify autoreactive B cell clones and exclude them from the repertoire by, for instance, clonal deletion or editing of the variable domain. Failure of tolerance mechanisms can ultimately lead to autoimmune diseases. Many autoimmune diseases are responsive to B cell-targeting therapies and are accompanied by disease-specific autoantibodies. Serological studies have provided insights into the association of autoantibodies with disease development, progression, or treatment. Autoantibodies are widely used as biomarkers for diagnosis and prognosis, for disease classification, and for guiding treatment choices. However, insights into the extent of autoantibody repertoires, i.e., the extent of tolerance failure, are lacking due to the limitations of the currently applied technologies. The study introduces a method that enables the study of plasma antibody repertoires at the protein level with molecular resolution. This LC-MS-based Fab profiling approach selectively generates IgG1 Fab fragments from affinity enriched plasma IgG and analyzes these Fab molecules by LC-MS, thereby resolving the diversity of polyclonal antibody mixtures and repertoires based on the unique mass and retention time of each Fab molecule. The application of this approach revealed that plasma as well as virus-specific IgG1 repertoires are unique and polyclonal, with a few clones showing particularly high abundances. This diversity of repertoires against infectious agents may, however, differ from that of autoreactive antibody repertoires as a result of the exclusion of autoreThis study introduces an antigen-specific liquid chromatography-mass spectrometry (LC-MS)-based IgG1 Fab profiling approach to analyze the molecular resolution of human autoantibody repertoires in rheumatoid arthritis (RA). The method uses the anti-citrullinated protein antibody (ACPA) repertoire as an example. The results show that each RA patient has a unique and diverse ACPA IgG1 repertoire dominated by a few antibody clones. In contrast to the total plasma IgG1 repertoire, the ACPA IgG1 sub-repertoire is characterized by an expansion of antibodies with one, two, or more Fab glycans, and different glycovariants of the same clone can be detected. The study indicates that the autoantibody response in RA is complex, unique to each patient, and dominated by a relatively low number of clones. Antibodies play a central role in protecting the host from pathogens, with each antibody harboring a highly variable region in its antigen-binding fragments (Fab). This variable domain is generated by recombination of variable (V), diversity (D), and joining (J) gene segments. The various processes may eventually give rise to billions of unique antibodies, including various clones capable of binding the same antigen. This high diversity and flexibility of the antibody repertoire allows antigen-specific immune responses even against newly arising or continuously evolving pathogens. To prevent the formation of antibodies that target the body itself, various tolerance mechanisms are in place. These mechanisms identify autoreactive B cell clones and exclude them from the repertoire by, for instance, clonal deletion or editing of the variable domain. Failure of tolerance mechanisms can ultimately lead to autoimmune diseases. Many autoimmune diseases are responsive to B cell-targeting therapies and are accompanied by disease-specific autoantibodies. Serological studies have provided insights into the association of autoantibodies with disease development, progression, or treatment. Autoantibodies are widely used as biomarkers for diagnosis and prognosis, for disease classification, and for guiding treatment choices. However, insights into the extent of autoantibody repertoires, i.e., the extent of tolerance failure, are lacking due to the limitations of the currently applied technologies. The study introduces a method that enables the study of plasma antibody repertoires at the protein level with molecular resolution. This LC-MS-based Fab profiling approach selectively generates IgG1 Fab fragments from affinity enriched plasma IgG and analyzes these Fab molecules by LC-MS, thereby resolving the diversity of polyclonal antibody mixtures and repertoires based on the unique mass and retention time of each Fab molecule. The application of this approach revealed that plasma as well as virus-specific IgG1 repertoires are unique and polyclonal, with a few clones showing particularly high abundances. This diversity of repertoires against infectious agents may, however, differ from that of autoreactive antibody repertoires as a result of the exclusion of autore