Asthma was significantly associated with elevated IgG antibody levels to C. pneumoniae, and this association was strongest for nonatopic longstanding asthma.

Halme S, von Hertzen L, Bloigu A, Kaprio J, Koskenvuo M, Leinonen M, Saikku P, Surcel H-M. Chlamydia pneumoniae-specific cell-mediated and humoral immunity in healthy people. Scand J Immunol 1998;47:517-520 In this work, cell-mediated immunity to Chlamydia pneumoniae was studied in 157 healthy individuals using lymphoproliferative assay and serum antibodies were analysed by microimmunofluorescence techniques. The C. pneumoniae-specific IgG antibodies were elevated more frequently and the geometric mean titres for IgG (67.5 versus 44.1; P ¼ 0.05) and IgA (14.9 versus 11.3; P ¼ 0.025) antibodies were significantly higher in males than in females. However, no gender-dependent differences were observed in cellular reactivity to C. pneumoniae, since the median cellular responses were similar (stimulation indices 7.5) in men and women. Although the cell-mediated and humoral responses to C. pneumoniae did not correlate clearly, elevated IgG antibodies were associated with slightly higher lymphocyte proliferation in comparison to all subjects (15.5 versus 7.5) and significantly stronger in comparison to those with persistently elevated IgA (> 80) antibodies (15.5 versus 3.5; P ¼ 0.023). Further studies are needed to evaluate a possible role of reduced cellular reactivity in the cause of chronic C. pneumoniae infection.