Complement-fixing (CF) antibodies against brain tissue antigens were demonstrated in seven cases of Mycoplasma pneumoniae infection with neurological complications. However, forty-three out of fifty-four cases (80%) without neurological symptoms also acquired antibodies to brain during the course of M. pneumoniae infection. 75% of the patients with antibodies to brain also had antibodies against human lung and liver, but the antibody titres to the latter tissue antigens were usually lower than against brain. The reactivity of M. pneumoniae antisera to brain and other tissues was significantly reduced by prior absorption with M. pneumoniae antigen, indicating the presence of a related antigen in M. pneumoniae organisms and these tissues. Fractionation by density gradient centrifugation of thirty-three sera from twenty-six cases showed the antibodies to brain to be of the IgM class exclusively in twenty-four cases while the CF antibodies to M. pneumoniae were associated with both IgM and IgG.
The brain antigen reacting in the CF test with M. pneumoniae antisera could be extracted with chloroform–methanol in the same way as the crude lipid CF antigen of M. pneumoniae.