The ABO blood group system is the most important blood type system (or blood group system) in human blood transfusion. Found on platelets, epithelium, and cells other than  erythrocytes, AB antigens (as with other serotypes) can also cause an adverse immune response to organ transplantation. The associated anti-A and anti-B antibodies are usually IgM antibodies, which are produced in the first years of life by sensitization to environmental substances, such as food, bacteria, and viruses. ABO blood types are also present in some other animals, for example rodents and apes, such as chimpanzees, bonobos, and gorillas.