We measured serum levels of IgG subclasses in 100 healthy adult humans and 64 patients with respiratory infections by utilizing the enzyme linked immunosorbent assay (ELISA). The patients were composed of 18 patients with acute bacterial pneumonia and bacterial infection of 9 patients with pulmonary emphysema, 27 patients with chronic bronchitis and 10 patients with bronchiectasis. In healthy adults, serum levels of IgG1 subclass decreased in proportion of age increase. Serum levels of IgG2 rose after 30 years of age. Serum levels of IgG3 and IgG4 showed no remarkable changes by age. In patients with respiratory infectious disease, serum levels of IgG1, IgG2 and IgG4 decreased significantly but IgG3 increased significantly. We also measured serum levels of IgG subclasses in 10 patients with acute bacterial pneumonia and bacterial infection of 5 patients with pulmonary emphysema, 12 patients with chronic bronchitis and 4 patients with bronchiectasis at both infected and convalescent phases. The serum levels of IgG2 in patients with pneumonia and pulmonary emphysema at convalescent phase were significantly lower than those in the patients of infected phase. Other subclasses showed no significant change. We summarized that IgG2 was consumed at the infected phase by protecting against bacterial infections. IgG2 probably has an important role of protecting against bacterial respiratory infections among all IgG subclasses.
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