The Human Blood Group System classifies blood into different groups based on the presence or absence of specific antigens on the surface of red blood cells. The most well-known systems are the ABO blood group and the Rh factor. The ABO system includes four main blood types: A, B, AB, and O, determined by the presence of A and B antigens. The Rh factor indicates whether blood is positive or negative, depending on the presence of the Rh antigen. Understanding blood groups is crucial for safe blood transfusions, organ transplants, and pregnancy, as incompatible blood types can lead to serious health complications.