The transfusion medicine (blood and blood product transfusion) is basically the study of antigen and antibodies to RBCs (red blood cells). The two most important factors of blood transfusion are ABO antigens and antibodies and Rh system, most important being the ABO blood group antigen system and Rh system is the second most important factor in transfusion medicine. Other factors such as transmission of infectious agents, immunologic and non immunologic reaction of blood and blood product transfusion are secondary to ABO blood group antigen system and Rh system.

What is the importance of ABO antigens and antibodies?

The ABO antigen system is the most important in blood and blood product transfusion (also called transfusion medicine) and was first recognized in the year 1900. The ABO antigens are present in the plasma and other body fluids as glycoprotein, although the ABO antigens are carbohydrates and attached to lipids glycosphingolipids or proteins (glycoprotein). H substance is the precursor on which antigen A and antigen B are attached. Addition of N-acetylgalactosamine forms antigen A and addition of galactose produces antigen B. individuals who can not form H substance due to lack of the gene form Bombay phenotype (Bombay blood group).

There are 2 basic antigens antigen A and antigen B in the ABO antigen system and based on these 2 antigens 4 major blood groups are recognized and they are A, B, AB and O. Blood group A contain antigen A, group B contains antigen B, AB group contain both antigen A and B and O group is absence of antigen A or B. There are also some rare blood groups such as Bombay blood group.

The importance of ABO antigen system is due to the fact that all the individuals produce antibodies A B carbohydrates which they lack (e.g. if an individual lack antigen A or A carbohydrate, he/she will produce antibodies against antigen B or anti-B and if an individual lack antigen B, he/she will produce antibodies against antigen A or anti-A and these anti-A and anti-B are termed isoagglutinins). No isoagglutinin (antibody against A or B) is present in AB blood group as they contain both antigens A and B. Individuals with blood group “O” contain both isoagglutinins (anti-A and anti-B) as they do not have antigens A or B.

Individuals with blood group AB are also called “universal recipients” as they do not contain any antibody against ABO phenotype and individuals with blood group O are also called “universal donors” as they can donate blood to all blood groups as they do not contain any antigen, A or B. But these terms are only for theoretical purpose and practically not followed.

Individuals with blood group “O” may be more susceptible to infections such as Candida albicans, Neisseria meningitidis, Streptococcus pneumoniae, Haemophilus influenzae etc. due to absence of antigen A and B, as these organisms may bind to polysaccharides on cells and antigens (A and B) may prevent/block the binding.

What is the importance of Rh system?

Rh system is the second most important system in transfusion medicine. Rh antigens are found on the membrane protein in RBCs. There are more than 40 different Rh antigens and D antigen is the most important. Presence of D antigen in an individual makes “Rh positive” and absence of D antigen makes an individual “Rh negative”.

D antigen is present in approximately 85% of individuals and absent in the remaining 15% of the individuals. D antigen is very potent antigen and exposure of small quantity of D alloantigen, by blood transfusion of by pregnancy in Rh negative individual may lead to production of anti-D alloantibody and this may lead to serious hemolysis in the recipient.

Pretransfusion testing:

Pretransfusion testing includes testing for ABO blood group (if the recipient of blood is of blood group A, he/she should receive ideally only group A blood), Rh factor (whether positive or negative and an Rh negative recipient should only receive Rh negative blood and Rh positive person should receive only Rh positive blood), cross-matching etc. Some tests are also compulsory (depending on the country) such as testing for HIV, hepatitis B and other viral diseases which are/can transmitted through blood.

Cross-matching of blood should be done, especially if the patient will have to receive packed red blood cells (PRBC) or whole blood (commonly used in developing countries and cross matching is routinely done). Non reactive cross-matching indicate absence of major incompatibility and fit for transfusion.

Strict implementation of pretransfusion screening of blood and blood products has virtually eliminated the risk of major incompatibility and transmission of infectious agents (virus, like HIV, hepatitis B, HTLV-1 etc. and parasites).