[Hematological diseases caused by excessive complement activation]

The complement system, which plays a key role in innate immunity, facilitates the elimination of foreign and endogenous substances in the body. Consequently, while a balance between complement activation and regulation is typically maintained, a disruption of this balance can lead to various diseases. Paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome (aHUS) are diseases in which abnormal complement regulatory factors cause excessive complement activation. To accurately diagnose diseases such as aHUS, international standard complement tests including activation markers such as Ba and sC5b-9 are used. The Japanese Society for Complement Research has contributed to the standardization of the International Complement Society and has established this testing system in Japan. In the present study, we revealed that the Ba level serves as a good predictive marker for the onset of thrombotic microangiopathy, a complication that typically occurs following hematopoietic stem cell transplantation. We also showed that complement activation levels rise more frequently postpartum than during pregnancy. Considering the growing number of drugs targeting the complement pathway, it is speculated that these complement markers will become available for testing in clinical settings in Japan.