The importance of humoral immunity against Rickettsia tsutsugamushi infection is largely unknown. As other infections by intracellular pathogen, it is generally believed that cellular immunity, including T-lymphocyte and macrophage, perform the
primary
role for the protection against scrub typhus. But there are several experimental evidences that antibody response do a some role at least in homologous infection, and antibody can inhibit the infection of cultured cells. The 56-kDa protein is
most
abundant proteins on rickettsial surface and known to be a major target antigen of host immune response. So we made rabbit hyperimmune sera to the recombinant 56-kDa protein produced in Escherichia coli and tested the neutralizing effect of these
sera
to inhibit the invasion of rickettsia on L cell monolayer. We used indirect immunofluorescent staining to count the number of rickettsial particle in the L cell, because conventional methods, such as plaque reduction assay and direct Giemsa
staining,
required too much time and are very difficult to count. With this new method, we found antibody against 56-kDa protein can inhibit the invasion/attatchment of Rickettsia tsutsugamushi into L cell.
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