Human blood types are classified into four main groups: A, B, AB, and O. These groups are determined by the presence or absence of specific antigens on the surface of red blood cells. Each blood type can also be classified as either Rh-positive or Rh-negative, depending on the presence of the Rh factor. Blood type compatibility is crucial for blood transfusions and organ transplants. For example, individuals with type O blood are considered universal donors, while those with type AB blood are universal recipients. Understanding blood types helps ensure safe medical procedures and effective treatment.