Blood types

Erythrocytes are labeled with cell surface antigens that determine blood type. Two types of inherited antigens are found on red blood cells: A antigens and B antigens. Accordingly, four blood types are possible (see Table 16.1):

Antibodies are specialized molecules produced by the immune system to attack foreign antigens. Therefore, an individual with type A blood produces

Table 16.1 Summary of ABO Blood Type System

Possible

Antibodies

Possible

Frequency in

Blood Type

genotypes

produced

transfusions

U.S.

A

AA, AO

anti-B

A, O

41%

B

BB, BO

anti-A

B, O

10%

AB

AB

none

A, B, AB, O

4%

O

OO

anti-A, anti-B

O

45%

anti-B antibodies, which attack type B antigens. An individual with type B blood produces anti-A antibodies, which attack type A antigens. Consequently, mixing incompatible blood can cause red blood cell destruction. The antibodies produced against a foreign blood type may cause agglutination (clumping) or hemolysis (rupture) of the donated erythrocytes.

Type AB blood contains A and B antigens on the red blood cells. Therefore, individuals with this blood type produce neither anti-A nor anti-B antibodies and can receive a transfusion of any blood type. Individuals with type AB blood are referred to as universal recipients.

Type O blood contains no antigens on the cell surface. In this case, any antibodies that the transfusion recipient may produce (anti-A or anti-B antibodies) have no antigens to attack. Therefore, no immune response against this blood exists. Individuals with type O blood are referred to as universal donors because this blood is suitable for transfusion in all individuals.

Another type of cell surface antigen found on red blood cells is the Rh factor. Red blood cells that contain the Rh factor are referred to as Rh-positive and RBCs without this factor are referred to as Rh-negative. This antigen also stimulates antibody production. Therefore, Rh-negative individuals that produce anti-Rh antibodies should receive only Rh-negative blood. Rh-positive individuals that do not produce anti-Rh antibodies can receive Rh-negative or Rh-positive blood. Approximately 85% of Caucasians are Rh-positive and 15% are Rh-negative. Over 99% of Asians, 95% of American blacks, and 100% of African blacks are Rh-positive.

Rh incompatibility may occur when an Rh negative mother carries an Rh-positive fetus. At the time of delivery, a small amount of the baby's Rh-positive blood may gain access to the maternal circulation. In response, the immune system of the mother produces anti-Rh antibodies. During the subsequent pregnancy, the fetus is exposed to these antibodies as they cross the placenta. If this fetus is also Rh-positive, then the anti-Rh antibodies attack the fetal erythrocytes and cause hemolytic disease of the newborn (eryth-roblastosis fetalis). This may occur in about 3% of second Rh-positive babies and about 10% of third Rh-positive babies. The incidence continues to increase with subsequent pregnancies.

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