A blood group is a classification of blood based on the presence or absence of specific antigens on the surface of red blood cells. The most well-known blood group systems are the ABO system and the Rh factor. The ABO system includes four main types: A, B, AB, and O, which are determined by the presence of A and B antigens. Blood groups are important for blood transfusions, organ transplants, and pregnancy. Mismatched blood transfusions can lead to serious complications, so knowing a person's blood group is crucial for safe medical procedures. Blood typing tests are used to determine an individual's blood group.