The human body has four main blood types: A, B, AB, and O.
This description provides an overview of the four main blood types found in humans: A, B, AB, and O. It explains how each type is determined by the presence or absence of certain antigens on the surface of red blood cells, and how the different types can affect blood transf
The ABO blood group system is used to denote the presence of one, both, or neither of the A and B antigens on erythrocytes. For human blood transfusions, it is the most important of the 48 different blood type classification systems currently recognized by the International Society of Blood Transfusions (ISBT) as of June 2025. A mismatch in this serotype can cause a potentially fatal adverse reaction after a transfusion, or an unwanted immune response to an organ transplant. Such mismatches are rare in modern medicine. The associated anti-A and anti-B antibodies are usually IgM antibodies, produced in the first years of life by sensitization to environmental substances such as food, bacteria, and viruses.