RhD refers to the D antigen present on the surface of red blood cells in the Rh blood group system. The human Rh system is divided into two categories according to whether or not the red blood cells contain the D antigen: those containing the D antigen are Rh-positive and those not containing the D antigen are Rh-negative. The D antigen in the Rh blood group has strong antigenic properties, so it can stimulate the body to produce the corresponding antibodies. Rh-negative people will produce antibodies against the D antigen after being exposed to Rh-positive blood, and if they are exposed to Rh-positive blood for the second time, a serious antigen-antibody reaction will occur, leading to red blood cell lysis and destruction. Therefore, Rh-negative people can only be transfused with Rh-negative blood, and serious transfusion reactions will occur if Rh-positive blood is introduced.