Therapeutic monoclonal antibodies have revolutionized the treatment of various inflammatory diseases. Immunogenicity against these antibodies has been shown to be clinically important: it is associated with shorter response duration because of diminishing concentrations in the blood and with infusion reactions. Concomitant immunomodulators in the form of methotrexate or azathioprine reduced the immunogenicity of therapeutic antibodies in rheumatoid arthritis, Crohn disease, and juvenile idiopathic arthritis. The occurrence of adverse events does not increase when immunomodulators are added to therapeutic antibodies. The mechanism whereby methotrexate and azathioprine influence immunogenicity remains unclear. Evidence-based consensus on prescribing concomitant immunomodulators is needed.