Blood group systems are classifications of blood based on the presence or absence of specific antigens on the surface of red blood cells. The most well-known blood group system is the ABO system, which categorizes blood into four main types: A, B, AB, and O. Each type has different antigens, which can affect blood transfusions and organ transplants. Another important blood group system is the Rh system, which determines whether blood is Rh-positive or Rh-negative based on the presence of the Rh factor antigen. Understanding these blood group systems is crucial for safe medical practices, as incompatible blood transfusions can lead to serious health complications.