It’s In The Blood

Blood groups
We’ve already seen what a complicated substance blood is (see our blog “A Bloody Miracle”), but did you know that there are some 32 blood groups and that over 600 blood-group antigens have been identified? Some of these are very rare, and some are largely restricted to certain ethnic groups.  Which blood group are you?

We’ve probably all heard of the ABO blood group classification system but what exactly is a blood group and why are they important? A blood type or group is a classification system based on the presence or absence of inherited antigenic substances on the surface of our red blood cells (RBCs) or erythrocytes.

These antigens can cause a severe immune reaction if a person is given incompatible blood during a transfusion, for instance. The recipient’s immune system is likely to kick into action, causing an acute hemolytic reaction with hemolysis (RBC destruction), renal failure and shock, and the possibility of death

Such antigens may be proteins, carbohydrates, glycoproteins, or glycolipids, depending on the blood group system. Of the 32 human blood group systems now recognized by the International Society of Blood Transfusion the two most important are ABO and the RhD antigen; these determine an individual’s blood type (A, B, AB and O, with + and − denoting RhD status).

ABO system

The ABO system is the most important blood-group system in human-blood transfusion. The associated anti-A and anti-B antibodies are usually immunoglobulin M (abbreviated IgM) antibodies. ABO IgM antibodies are produced in the first years of life by sensitization to environmental substances such as food, bacteria and viruses.

A quick look at the AB group will explain how the system works: blood group AB individuals have both A and B antigens on the surface of their RBCs, while their blood plasma does not contain any antibodies against either A or B antigen. Therefore, an individual with type AB blood can receive blood from any group (with AB being preferable), but cannot donate blood to any group other than AB. They are known as universal recipients. Conversely, blood group O individuals are known as universal donors because they do not have either A or B antigens on the surface of their RBCs, but their blood serum contains IgM anti-A and anti-B antibodies against the A and B antigens. Although group O individuals can receive blood only from a group O individual, they can donate blood to individuals of any ABO blood group, which is useful if donors are needed for an emergency transfusion.

Rh blood group system Blood typing with Rhesus factor

The Rh system (where Rh stands for Rhesus because of the early research on rhesus macaque serum) is the second most significant blood-group system with some 50 antigens currently known. Of the five main Rh antigens, the most significant is the D antigen, because it is the one most likely to provoke an immune system response.

Blood transfusions

Patients should ideally receive their own blood or type-specific blood products to minimize the chance of a transfusion reaction although this may not always be possible in an emergency. Risks can be reduced by cross-matching blood; this involves mixing a sample of the recipient’s serum with a sample of the donor’s red blood cells and checking if the mixture agglutinates (forms clumps). Where this is not immediately obvious, technicians usually check for agglutination under a microscope. Blood donors with particularly strong anti-A, anti-B or any atypical blood group antibody are excluded from being able to give blood.

Problems during pregnancy

Many pregnant women carry a fetus with a blood type that is different from their own. In such cases, the mother can form antibodies against the developing child’s RBCs. Sometimes these maternal antibodies in the form of IgG (a small immunoglobulin) can cross the placenta and cause hemolysis of fetal RBCs, which in turn can lead to hemolytic disease of the newborn known as erythroblastosis fetalis. This is a condition in which there are mild to severe low fetal blood counts and it can be lethal.

Where a pregnant woman is known to have anti-D antibodies, the Rh blood type of the fetus can be tested by analysis of fetal DNA in maternal plasma to assess the risk. A major advance of 20th century medicine was to treat this disease by stopping the formation of Anti-D antibodies by D negative mothers with an injectable medication called Rho(D) immune globulin.

Changing blood groups

Did you know that your blood group can sometimes change? Very rarely an individual’s blood type changes through addition or suppression of an antigen perhaps because of infection or autoimmune disease, or where there has been a bone marrow transplant. If the recipient obtains marrow from someone with a different ABO type, their blood type will eventually convert to that of the donor.

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