Varying levels of cross-reactivity to some mycoplasma species were observed in the sera of patients infected with Mycoplasma pneumoniae and even in normal human sera by enzyme-linked immunosorbent assay (ELISA). The absorption of the patients' sera with M. pneumoniae lysate showed the decrease in ELISA titers not only to M. pneumoniae, but also to other mycoplasma species. These results suggested the existence of cross-reactive antibodies to mycoplasmas in human sera. Cross-reactive antibodies to M. pneumoniae and other mycoplasmas in the patients' sera were also demonstrated by Western blotting technique.The enzyme-linked immunosorbent assay (ELISA) has been used for routine serological diagnosis of Mycoplasma pneumoniae infection because of its high sensitivity and simplicity (4,5,7,19,23,24). But, as high background readings are often observed in the ELISA, the interpretation is often very difficult. It is suspected that the main factor causing background readings is the nonspecific adsorption of serum and the conjugate on the surface of plate wells or on M. pneumoniae antigens.We previously showed by the ELISA that varying levels of cross-reactivity to some mycoplasma species were seen in human normal immunoglobulin G (human normal IgG) prepared from pooled plasma of at least 500 normal human donors and in some human sera not associated with M. pneumoniae infections (21). Furthermore, we also showed that the nonspecific adsorption of human normal IgG on the surface of plate wells and on medium components which might contaminate in M. pneumoniae antigen could be disregarded (21). These results suggested the existence of the cross-reactive antibodies to mycoplasmas that lowered the specificity of the ELISA in serodiagnosis of M. pneumoniae infection.In this study, we investigated by ELISA and immunoblotting technique the characterization of the cross-reactive antibodies found in sera of patients infected with M. pneumoniae.

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