Autoantibodies have been associated with human pathologies for a long time, 
particularly with autoimmune diseases (AIDs). Rheumatoid factor (RF) is known 
since the late 1930s to be associated with rheumatoid arthritis (RA). The 
discovery of anticitrullinated protein antibodies in the last century has 
changed this and other posttranslational modifications (PTM) relevant to RA have 
since been described. Such PTM introduce neoepitopes in proteins that can 
generate novel autoantibody specificities. The recent recognition of these novel 
specificities in RA provides a unique opportunity to understand human B-cell 
development in vivo. In this paper, we will review the three of the main classes 
of PTMs already associated with RA: citrullination, carbamylation, and 
oxidation. With the advancement of research methodologies it should be expected 
that other autoantibodies against PTM proteins could be discovered in patients 
with autoimmune diseases. Many of such autoantibodies may provide significant 
biomarker potential.