Coreactivity of Legionella pneumophila immune sera in the Mycoplasma pneumoniae complement fixation test.

Sheep and guinea pigs were immunized with cellular and extracellular antigen from Legionella pneumophila bacteria. After immunization, the animals developed immunoglobulin G titers against the immunizing agent. The same sera were also tested in a Mycoplasma pneumoniae complement fixation test. All the preimmunization sera from sheep showed positive M. pneumoniae complement fixation tests of varying titers, with significant antibody rises in two of five sheep as a result of the Legionella immunizations. In contrast to the sheep, all the guinea pigs were negative in the M. pneumoniae complement fixation test, both in their preimmunization sera and after completion of the Legionella immunizations. The results obtained with the sheep sera may be explained as a nonspecific booster effect of Legionella bacteria upon previously elicited immune responses.