The present application provides the medicaments comprising the antibodies binding to phospholipase D4 (PLD4) as well as a method using said medicaments for detecting and suppressing activated B cells. The present application is further directed to therapy of auto-immune diseases and allergosis, resulting from the active-repressing function. In order to solves these problems, the present application provides that a monoclonal antibody binding to the extracellular domain of phospholipase D4 (PLD4) protein, or a fragment containing an antigen-binding region thereof as an active ingredient.
