CD40LG and spotted fever: Moreover, taking into account the antigen similarity between spotted fever group Rickettsia species, the similarity of the clinical symptoms of fevers caused by them, and the cross-reactivity between secreted immunoglobulins to different rickettsias of this group (15, –, 17), it may be hypothesized that detection of specific IgA in addition to IgM has an important diagnostic value for other tick-borne spotted fever rickettsioses distributed worldwide.