A blood type (also called a blood group) is a classification of blood based on the presence or absence of inherited antigenic substances on the surface of red blood cells (RBCs). These antigens may be proteins, carbohydrates, glycoproteins or glycolipids, depending on the blood group system, and some of these antigens are also present on the surface of other types of cells of various tissues. Several of these red blood cell surface antigens, that stem from one allele (or very closely linked genes), collectively form a blood group system.Immunological effects of mismatched blood transfusions are much more likely to involve components of the ABO blood group system or the RhD antigen (also known as the Rhesus factor or Rhesus D antigen) of the Rhesus blood group system than components of any of the other blood group systems; hence, in the routine preparation of blood for transfusion in a blood bank, the presence or absence the immunogenic blood group antigens, the A antigen, the B antigen and the RhD antigen are always determined for all recipient and donor blood. This identifies the ABO blood group and the RhD antigen status, which are both stated in the common terminology A positive, O negative, etc., where a capital letter (A, B or O) refers to the ABO blood group, and positive or negative refers to the presence or absence of the RhD antigen of the Rhesus blood group system. In the routine preparation and selection of donor blood for blood transfusion, it is not necessary to determine the status of any other blood group antigens or antibodies, because antibody screening and cross-matching (or computer aided simulated cross-matching) prior to transfusion detects if there are any other blood group incompatibilities between potential donor blood and intended recipients.If an individual is exposed to a blood group antigen that is not recognised as self, the immune system will produce antibodies that can specifically bind to that particular blood group antigen and an immunological memory against that antigen is formed. The individual will have become sensitized to that blood group antigen. These antibodies can bind to antigens on the surface of transfused red blood cells (or other tissue cells) often leading to destruction of the cells by recruitment of other components of the immune system. When IgM antibodies bind to the transfused cells, the transfused cells can clump. It is vital that compatible blood is selected for transfusions and that compatible tissue is selected for organ transplantation. Transfusion reactions involving minor antigens or weak antibodies may lead to minor problems. However, more serious incompatibilities can lead to a more vigorous immune response with massive RBC destruction, low blood pressure, and even death.Blood types are inherited and represent contributions from both parents. Often, pregnant women carry a fetus with a different blood type from their own, and sometimes the mother forms antibodies against the red blood cells of the fetus, which causes hemolysis of fetal RBCs, and which in turn can lead to low fetal blood counts, a condition known as hemolytic disease of the newborn. Some blood types are associated with inheritance of other diseases; for example, the Kell antigen is associated with McLeod syndrome. Certain blood types may affect susceptibility to infections, an example being the resistance to specific malaria species seen in individuals lacking the Duffy antigen. The Duffy antigen, presumedly as a result of natural selection, is less common in ethnic groups from areas with a high incidence of malaria.The two most significant blood group systems were discovered during early experiments with blood transfusion: the ABO group in 1901 and the Rhesus group in 1937. Development of the Coombs test in 1945, the advent of transfusion medicine, and the understanding of hemolytic disease of the newborn led to discovery of more blood groups. Today, a total of 29 human blood group systems are recognized by the International Society of Blood Transfusion (ISBT). A complete blood type would describe a full set of 29 substances on the surface of RBCs, and an individual's blood type is one of the many possible combinations of blood group antigens. Across the 29 blood groups, over 600 different blood group antigens have been found, but many of these are very rare or are mainly found in certain ethnic groups. Almost always, an individual has the same blood group for life; but very rarely, an individual's blood type changes through addition or suppression of an antigen in infection, malignancy or autoimmune disease. Blood types have been used in forensic science and in paternity testing, but both of these uses are being replaced by DNA analysis, which provides greater certitude.