Human blood group systems

The International Society of Blood Transfusion (ISBT) currently recognises 30 major blood group systems (including the AB and Rh systems). Thus, in addition to the ABO antigens and Rhesus antigens, many other antigens are expressed on the red blood cell surface membrane. For example, an individual can be AB RhD positive, and at the same time M and N positive (MNS system), K positive (Kell system), and Lea or Leb positive (Lewis system). Many of the blood group systems were named after the patients in whom the corresponding antibodies were initially encountered.

The ISBT definition of a blood group system is where one or more antigens are "controlled at a single gene locus or by two or more very closely linked homologous genes with little or no observable recombination between them".

Blood grouping postulates
Blood is composed of cells suspended in a liquid. The liquid portion is the plasma, from which therapeutic fractions and derivatives are made.

Suspended in the plasma are three types of cells:
 * Red cells carry oxygen
 * White cells fight infection
 * Platelets stop bleeding in injuries

The most common type of grouping is the ABO grouping. The varieties of protein coating on red blood cells divides blood into four groups:


 * A (A oligosaccharide is present)
 * B (B oligosaccharide is present)
 * AB (A and B oligosaccharides are present)
 * O (neither A nor B, only their precursor H oligosaccharide present)

There are subtypes under this grouping (listed as A1, A2, A1B or A2B…) some of which are quite rare. Apart from this there is a protein which plays an important part in the grouping of blood. This is called the Rh factor. If this is present, the particular blood type is called positive. If it is absent, it is called negative. Thus we have the following broad categories:
 * A2 Negative (A1 -ve)
 * A1 Positive (A1 +ve)
 * A1B Negative (A1B -ve)
 * A1B Positive (A1B +ve)
 * A2 Negative (A2 -ve)
 * A2 Positive (A2 +ve)
 * A2B Negative (A2B -ve)
 * A2B Positive (A2B +ve)
 * B Negative (B -ve)
 * B Positive (B +ve)
 * B1 Positive (B1 +ve)
 * O Negative (O -ve)
 * O Positive (O +ve)

Rare blood types
In the "ABO" system, all blood belongs to one of four major groups: A, B, AB, or O. But there are more than two hundred minor blood groups that can complicate blood transfusions. These are known as rare blood types. Whereas common blood types are expressed in a letter or two, with maybe a plus or a minus, a smaller number of people express their blood type in an extensive series of letters in addition to their 'AB-' type designation.