Classify, as blood is a term used to describe the process of grouping blood into different types based on the presence or absence of certain antigens. Blood typing is an important part of medical care, as it helps to ensure that the right TYPE of blood is given to a patient during a transfusion. The four main blood types are A, B, AB, and O, and each TYPE is further divided into positive and negative subtypes. Knowing a person's blood TYPE can also help to determine their risk for certain diseases and conditions.