Human granulocytic anaplasmosis (HGA), an increasingly recognized febrile 
tick-borne illness, is caused by a gram-negative obligate intracellular 
bacterium Anaplasma phagocytophilum. Because of nonspecific clinical 
manifestations, diagnosis of HGA highly depends on laboratory tests. 
Identification of immunoreactive proteins is prerequisite for development of 
specific and sensitive immunoassays for HGA. In this study, we identified novel 
immunoreactive proteins of A. phagocytophilum. Previous studies indicated that 
secreted proteins of A. phagocytophilum and other bacteria can be immunoreactive 
antigens. Here we in silico screened A. phagocytophilum genome for encoding 
proteins which bear features of type IV secretion system substrates. Among 
seventy seven predicted proteins, fourteen proteins were determined for 
antigenicity and nine proteins were showed to be immunoreactive antigens. In 
addition, an APH1384 peptide harboring a B cell epitope predicted by 
bioinformatics was found specifically reacting with anti-A. phagocytophilum 
sera. Hereby, we identified novel immunoreactive proteins and delineated a 
specific epitope of A. phagocytophilum, which might be employed for HGA 
diagnosis.