Background: Tuberculosis (TB) is a chronic bacterial infection caused by M. Tuberculosis. Studies of antibody response in TB have focussed mainly on their usefulness as a diagnosticserological tool, with little attention given to analysis of antibodies at the isotype and subclass levelin relation to disease pathogenesis. Hence the present study was done to analyse IgG subclass responseat different stages of tuberculosis, in order to understand the immunological events associated withdisease development. Material/Methods: Sera samples were collected from 104 subjects: 79 tuberculosispatients (fresh, relapse and chronic cases) and 25 healthy normals. IgG subclass antibody response wasanalysed by indirect plate peroxidase ELISA against previously reported mycobacterial serine protease(ES-31) antigen. Results: Fresh cases of tuberculosis showed increased IgG1 and IgG3 antibodies, whilea few cases showed moderately increased IgG2. IgG1 and IgG3 were found to be elevated with increasedbacillary load. Relapse and chronic cases showed increased IgG1 and IgG3, while positivity to IgG2 wasdecreased. Chronic cases showed a moderate increase in IgG4 antibody. Thus IgG1 and IgG3 were predominantin all forms of tuberculosis. Conclusions: The elevated levels of IgG1 and IgG3 antibodies to mycobacterialserine protease in active tuberculosis observed in this study provide an additional marker for diagnosisof tuberculosis. Furthermore, the higher level of these antibodies with high bacillary load patientsand in chronic cases of tuberculosis may provide valuable insight into their possible role in diseaseprogression.

Keywords: Antibody Formation, Antibodies, Bacterial - blood, Biological Markers, Disease Progression, Immunoglobulin G - blood, Mycobacterium tuberculosis - immunology, Serine Endopeptidases - immunology, Tuberculosis, Pulmonary - diagnosis