Blood Groups

Thirty-three major blood group systems (including the AB and Rh systems) were recognised by the International Society of Blood Transfusion (ISBT) in October 2012.^[1] 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 Le^a or Le^b positive (Lewis system). Many of the blood group systems were named after the patients in whom the corresponding antibodies were initially encountered.

Blood is composed of cells suspended in a liquid like substance. The liquid portion is the plasma. 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 glycoprotein 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:^[3]

• A1 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)