BLOOD TYPE
Till now a total of 29 human blood group systems have been identified by the International Society of Blood Transfusion (ISBT). There are many types of red blood cells and a person can have red blood cells of only one type. Blood groups are determined by the presence or absence of chemical substances on the membrane surface of red blood cells. These substances contain specific sequences of particular amino acids and carbohydrates. These chemical substances are called recognition markers or antigens. The ABO group was discovered in 1901 and the Rhesus group, was discovered in 1937 during early experiments with blood transfusion. These two blood groups are reflected in the common nomenclature A positive, O negative, etc. with letters referring to ABO group and positive/negative to the presence of the RhD antigen of the Rhesus group. If only marker A alone is present, the person is said to have Type A blood. If only marker B is present, the blood is of Type B. If neither A nor B markers are present, the blood is Type O. If both the A and B markers are present, the blood is Type AB. The Rh marker is present or absent regardless of the presence of A and B markers. If Rh factor is present, the blood is said to be Rh positive and if it is absent, the blood is said to be Rh negative. The most common type of blood type found among human beings is A positive. A positive blood has an A marker and also an Rh marker.

Blood types are genetically inherited from parents. Some blood types are extremely uncommon, and may be found only in certain ethnic groups. Some blood types are connected with inheritance of certain diseases. For example, an antigen called the Kell antigen is associated with McLeod syndrome. Some blood types give additional protection from certain diseases. People with such blood types are more immune to those diseases. For example, an antigen called the Duffy antigen gives partial resistance to malaria. Sometimes a person's blood type may change through the addition or suppression of an antigen during infection, but this is extremely rare.

Blood typing is very important for many medical reasons. Blood typing finds application during blood transfusion. If a person loses a lot of blood during an accident or an operation, a blood transfusion may be required to replace some of the lost red blood cells. Since the human body has the tendency to produce antibodies against substances that are foreign, the blood that is being transfused must be matched so that these substances are not present in it. A person who is blood type A positive will not produce antibodies against the A or Rh markers but will generate antibodies against the B marker. If blood containing the B marker from types B positive, B negative, AB positive or AB negative is transfused into this person, the person's anti-B antibodies will rapidly attack and destroy transfused blood. For a successful blood transfusion into an A positive blood type individual, blood that is type O negative, O positive, A negative, or A positive is needed because these blood types will not be attacked by the patient's anti-B antibodies.

Persons having blood type O negative are called "universal donors," and those with type AB positive blood are called "universal recipients". The terms "universal donor" and "universal recipient" are not very useful, because they only consider the reaction of the patient's antibodies to received blood, and not the antibodies present in the transfused blood. Thus, although a transfusion of O negative blood to a patient of blood group A or B is unlikely to produce an immune reaction due to the recipient's antibodies, the transfused blood may itself contain antibodies to the patient's A and B antigens; this can cause an adverse reaction, although the risk is far less than that of an O negative patient receiving types A or B. For this reason, an exact ABO-type match is preferable where circumstances allow. Additionally, the other red blood cell surface antigens that belong to blood groups outside of the ABO convention might cause an adverse reaction, if they can bind the corresponding antibodies.