The Rh blood group system is a classification of blood based on the presence or absence of the Rh factor, a specific protein found on the surface of red blood cells. Individuals who have this protein are classified as Rh-positive, while those who do not are classified as Rh-negative. This system is important for blood transfusions and pregnancy, as mismatches can lead to serious health complications. The most common Rh factor is RhD, which is the primary focus in blood typing. If an Rh-negative person receives Rh-positive blood, their immune system may react against the foreign Rh factor, potentially causing a dangerous immune response. Understanding the Rh blood group system is crucial for safe medical practices.