ABO, Rh and the complete blood count

Antigens, antibodies and why groups matter

An antigen is a molecule that the immune system can recognise. Red blood cells carry antigens on their surface, and the plasma carries antibodies that bind antigens the person does not have. In the ABO system, antibodies are naturally present against the antigens the person lacks: someone with group A has anti-B antibodies, someone with group B has anti-A, and someone with group O has both. If red cells carrying an antigen meet the matching antibody, the cells clump together in a reaction called agglutination and are destroyed, releasing haemoglobin into the plasma. That reaction is why transfusion compatibility depends on matching antigens and antibodies. A small sample is always cross-matched before transfusion, because a mismatch can be rapidly fatal even when the volumes are small.

For example: Giving group A red cells to a group B patient is dangerous, because the patient’s anti-A antibodies bind the donor cells and cause them to clump and break down.

The ABO system

The ABO system is based on two antigens, A and B. Group A cells carry antigen A; group B cells carry antigen B; group AB cells carry both; group O cells carry neither. Plasma antibodies follow the opposite pattern: group A plasma has anti-B, group B plasma has anti-A, group AB plasma has neither, and group O plasma has both. This produces the compatibility rules. Group O red cells can be given to any ABO group because they carry neither antigen, which is why O is called the universal donor for red cells. Group AB plasma can be given to any group because it contains neither antibody. A person with group AB can receive red cells from any ABO group, so AB is called the universal recipient for red cells. The rules apply to the donor’s antigens and the recipient’s antibodies, not the other way round.

For example: A group O negative unit is often used in an emergency before the patient’s blood group is known, because it lacks A, B and Rh D antigens.

Common misconception: Universal donor and universal recipient describe red cell transfusion, not every blood product. Plasma compatibility follows the opposite logic.

The Rh system and pregnancy

The Rh system centres on the D antigen, also called the Rh factor. People whose red cells carry it are Rh-positive; those without it are Rh-negative. Unlike the ABO antibodies, anti-D antibodies are not normally present from birth; they appear only after an Rh-negative person is exposed to Rh-positive red cells, for example by transfusion or during childbirth. Once formed, they can cause a serious reaction on later exposure. This matters most in pregnancy. If an Rh-negative mother carries an Rh-positive fetus, a small amount of fetal blood may enter her circulation at delivery, and she may develop anti-D antibodies. In a later pregnancy with another Rh-positive fetus, those antibodies can cross the placenta and destroy the fetal red cells, causing haemolytic disease of the newborn. This is prevented by giving the mother anti-D immunoglobulin at the appropriate time.

For example: An Rh-negative woman is offered anti-D immunoglobulin after delivery of an Rh-positive baby, so that she does not become sensitised and harm a future pregnancy.

What a complete blood count measures

A complete blood count groups the main measurements of blood cells. The red cell measurements include the red blood cell count, the haemoglobin concentration and the haematocrit, which describe how much oxygen-carrying capacity the blood has. The mean corpuscular volume (MCV) reports the average size of the red cells, and the mean corpuscular haemoglobin (MCH) reports the average amount of haemoglobin per cell; these help identify the type of anaemia. The white cell measurements include the total white cell count and the differential, which reports the proportion of each type. The platelet count reports how many platelets are present. Together these values describe the quantity and quality of the formed elements, and they are interpreted with the patient’s symptoms, history and other tests rather than in isolation.

For example: A low haemoglobin with small red cells suggests iron deficiency, while a low haemoglobin with large red cells suggests vitamin B12 or folate deficiency.

Putting the values together

A blood count becomes useful when its values are read together. A low haemoglobin and a low haematocrit confirm reduced oxygen-carrying capacity; the MCV then suggests the cause: small cells point to iron deficiency or chronic blood loss, large cells to vitamin B12 or folate deficiency, and normal-sized cells to blood loss, kidney disease or chronic illness. A raised white cell count with a neutrophil predominance suggests bacterial infection, while a low count raises concern about the marrow or about overwhelming infection. A low platelet count increases bleeding risk; a high count increases clotting risk. Because each value can be affected by hydration, bleeding, infection and treatment, the count is best used to test a physiological prediction. Ask what the history suggests, predict which values should change, then check whether they do.

For example: A patient with heavy periods has fatigue, low haemoglobin and a low MCV. The pattern fits iron deficiency from chronic blood loss, so the cause is looked for as well as the deficiency treated.