CD40LG and Q fever: To investigate the possible clinical differences between acute Q fever with and without serum M. pneumoniae IgM, the 102 cases of acute Q fever listed in Table 1 were divided into 2 groups according to the results of the M. pneumoniae IgM in acute or convalescent phase serum for comparison (59 positive cases [57.8%] and 43 negative cases [42.2%]).