The D antigen is a specific protein found on the surface of red blood cells. It is a key component of the Rh blood group system, which classifies blood types based on the presence or absence of this antigen. Individuals who have the D antigen are considered Rh-positive, while those who do not are Rh-negative. The presence of the D antigen is important in blood transfusions and pregnancy. If an Rh-negative person receives Rh-positive blood, their immune system may react against the D antigen, leading to complications. Similarly, during pregnancy, an Rh-negative mother carrying an Rh-positive baby may develop antibodies against the D antigen, which can affect the baby’s health.