Complement activation in pulmonary tuberculosis.

The alterations in serum/plasma levels of total haemolytic complement activity (CH50), complement components C3 and C3d, and circulating immune complexes (CICs) in patients with pulmonary tuberculosis were analysed in relation to the severity of disease and treatment status. The mean levels of CH50, C3, C3d and CICs were significantly higher in untreated than treated patients and in normal controls. In the untreated group, the level of each of these four parameters except C3d was significantly higher in patients with far advanced disease than in those with moderately advanced disease, whereas the difference between treated patients and normal controls was not statistically significant for any of the four parameters tested. There were statistically significant correlations between levels of CICs and both C3 and C3d in the untreated tuberculosis patients. However, the correlations for the same parameters were not significant when treated patients were considered. The CH50 levels in tuberculosis patients suggest a functional classical complement pathway, which is essential for immune complex solubilisation. High C3d level in untreated patients is indicative of increased complement activation, which in turn shows significant correlation with levels of CICs. It appears that the intact and elevated complement proteins and their proper activation by CICs prevents tuberculosis from becoming a typical immune complex disease.