Blood types

Blood types

The surfaces of erythrocytes contain molecules that can be antigenic. The type of antigen present is determined by the genes. There are a number of different antigens on red blood cells but we will be discussing the AB and Rh antigens because they are the antigens that can cause the most problems with transfusion reactions.

You may be wondering why we are not concerned with antibodies present in the O- blood plasma causing clumping the recipient's cells.

This is not usually a problem because the amount of antibody in the donated plasma is diluted by the recipient's plasma. This means there is not enough antibody in any one place to cause much erythrocyte damage. Of course, if one uses matching blood not even this problem can occur.

most common blood type

Type O is the most common blood type (around 46%), followed by A, then B, and type AB is the rarest (around 4%).

If a woman is sensitized

her antibodies may affect an Rh+ fetus of a later pregnancy. If enough maternal anti-Rh antibody crosses placenta, some fetal RBCs destroyed. The initial sensitization of the woman can be prevented by giving the mother anti-Rh antibodies (of a type that do not cross the placenta) during the last months of pregnancy and during and after delivery of her first Rh+ child. "These antibodies bind to the antigenic sites on any Rh positive erythrocytes that might have entered the mother's blood during delivery and prevent them from inducing antibody synthesis by the mother."

As an example of a transfusion reaction

let's look at what would happen if type A blood is given to a person who has type O blood. The donated RBCs have antigen A on their surface. These donated RBCs are clumped by anti-A antibody in recipient's plasma. These clumped cells can block small blood vessels. Clumped RBCs also rupture and release hemoglobin. If much hemoglobin is released, hemoglobin can precipitate in the kidneys causing dangerous impairment of kidney function.

For the first Rh+ fetus

of an Rh-mother there is usually no problem for the fetus. None or very few fetal RBCs cross into the mother's blood during the pregnancy. However during birth, when the placenta separates from the uterine wall it is likely that enough fetal cells enter the mother's circulation to stimulate the mother to develop an immune response to the Rh+ antigen. (She develops antibodies and memory cells for the Rh antigen.)

If this sensitized person is later given Rh+ blood,

the person quickly produces high levels of anti-Rh antibody and these antibodies can cause clumping and destruction of the donated RBCs and the other symptoms of a transfusion reaction.

In an extreme emergency

type O- blood could be administered to anyone. Type O- RBCs contain no A, B, or Rh antigens and therefore will not be clumped by antibodies against A, B or Rh that may be in the recipient's plasma.

In summary,

we are all exposed to antigen A and B. We don't make antibodies to A or B if they are "self" (part of our own red blood cells). We do make antibodies to A or B if they are "foreign" (not present on our own red blood cells.)

The Rh negative person who is exposed to Rh antigen

will (as is usual in antibody-mediated immunity) in about 10 days have made antibodies against Rh and these antibodies will be circulating in the blood. This first exposure to Rh antigen usually has no observable ill effects because the development of antibody in response to initial exposure takes some time and the antibody levels are not that high. However the first exposure also stimulates the development of memory cells. These memory cells are primed to produce large quantities of antibody against Rh if the person is exposed again. The first exposure sensitizes the person to the Rh antigen.

About 85% of people are Rh positive,

About 85% of people are Rh positive, 15% are Rh negative. The only person who can develop antibodies to the Rh antigen is an Rh negative person. This can only happen if an Rh- person is exposed to Rh antigen. Such exposure might occur by transfusion of Rh+ blood or in some cases in which an Rh- mother is carrying an Rh+ fetus.

Hemolytic disease of the newborn

If cells from Rh+ fetus enter an Rh-mother's circulation the mother will be exposed to the Rh antigen and become sensitized.

Transfusions

Loss of blood can obviously be fatal. Historically it was observed that in the case of significant loss of blood, if blood from another person could quickly be introduced into the patient's circulation, the patient might be helped. In some cases, however, the blood itself caused reactions that produced damage or death. When the existence of blood types was discovered, the explanation of these damaging transfusion reactions became clear and it was then understood how transfusions could be done safely.

Why are there antibodies in plasma?

Not known. But it is observed that antibodies to A and B begin to appear a few months after birth. Certain common bacteria have molecules very similar to antigen A and B; antibodies made against these bacterial molecules also will bind to antigen A and B on red blood cells.

To avoid transfusion reactions

Select blood of the same ABORh type. Also check for any problems involving antigens other than A, B and Rh. "There are approximately 12 other human-erythrocyte antigen systems. Compatibility is determined by mixing the red blood cells from the potential donor with plasma from the recipient. If no clumping occurs, the blood is considered an adequate match for transfusion." 1