The authors examined the relationship between immune biomarkers and indoor air pollution cross-sectionally in school children 9-11 yr of age who had immunologically related respiratory diseases and who resided in Hungarian cities. Nitrogen dioxide, formaldehyde, benzene, xylene, and toluene were measured passively indoors prior to the collection of venous blood samples for blood counts and identification of immune biomarkers. House dust mite allergen was also measured. Numerous immune biomarkers were significantly elevated in these sensitive children, compared with normal children, and several biomarker alterations in these children were related to high concentrations of air pollutants in the home. The strongest and most significant associations were seen between high indoor nitrogen dioxide concentrations and increased white blood cells, monocytes, red blood cells, and immunoglobulin G (IgG), as well as decreased immunoglobulin M (IgM) and Klebsiella pneumoniae-specific IgM. Bacterial-specific IgGs were related significantly to formaldehyde concentrations. These findings suggest the important role of indoor air pollutants in immune reactions.