Blood Typing Lab

ABO Blood Typing Lab

Background

By volume, blood tissue is approximately 55% plasma and 45% cells. Plasma is a straw-colored liquid consisting of 90% water and a variety of salts and proteins important for maintaining osmotic balance, buffering against pH changes, maintaining blood viscosity, transporting certain materials, and for blood clotting when a blood vessel is injured.

There are three major types of blood cells—red blood cells (erythrocytes), white blood cells (leukocytes), and platelets.

• Red blood cells, the most numerous cells in the blood, carry oxygen from the lungs to all parts of the body. A red blood cell is a biconcave disk with a thin center. This shape provides a large surface area for diffusion of oxygen. Red blood cells contain the protein hemoglobin. Iron is incorporated into the hemoglobin molecule. When blood travels through the lungs, the oxygen in the lungs combines with the iron in hemoglobin. When the blood moves through the body’s capillary system, the oxygen carried in the red blood cells is released from the iron in hemoglobin to the other cells of the body. Red blood cells’ flexibility allows them to pass through even the smallest blood vessels.

• White blood cells make up only about 1% of the blood volume. They are an important part of the immune system. Their primary function is to provide defense against invaders in the body, which may include bacteria, parasites, fungi, and viruses. White blood cells may attack a foreign body directly, they may produce antibodies that identify, attach to, and neutralize a foreign body, or they may trigger other cells to act in destroying the foreign body.

• Platelets perform a vital function in the process of coagulation, or blood clotting, which occurs when a blood vessel is injured.

Blood Types

Although the basic composition and function of blood in each of us is the same, there are different human blood types. The cell membrane of red blood cells, like that of other cells, has molecules that project from its surface. Some of the molecules function as identification badges, allowing the immune system to recognize the cell as a normal component of an individual’s body. If blood from a person whose red cells have different surface molecules is injected into someone, those molecules are recognized as antigenic, or foreign to the body. The immune system attacks the antigens and attempts to destroy them and the cells that carry them. This is why transfusion with an incompatible blood type is harmful. The recipient’s body recognizes the antigens on the transfused red blood cells as foreign and attacks and destroys the cells. For that reason, donated blood is thoroughly tested for A, B, O, and Rh antigens and is transfused only into compatible recipients.

The ABO Blood Groups

The ABO blood groups (types) result from the presence or absence of two antigens, A and B, on the surface of the red blood cells. If antigens are present very early in life, the immune system recognizes those antigens as “self” and will not generate an immune response to them. As a result, the body does not generate antibodies to any A and B antigens present on its own blood cells. However, the immune system does produce antibodies to any A and B blood antigens not present on the organism’s own cells. Type A blood has the A antigen on its red blood cells and anti-B antibodies in the plasma. Type B blood has the B antigen on its red blood cells and A antibodies in the plasma. Type AB blood has both A and B antigens on the red blood cells and no antibodies in the plasma. Finally, Type O blood has neither A nor B antigens on the red blood cells and both A and B antibodies in the plasma.

These antibodies are present even if the person has not had any foreign blood introduced into their body . It is hypothesized that the antibodies are present because of similarity between the A and B blood antigens and other antigens present in the environment . If two antigens are similar enough, the antibodies generated to one antigen will also recognize the other.

The Rh Blood Groups

Another important antigen found on the surface of blood cells is the Rh factor . The Rh antigen is actually a whole group of closely related antigens . Blood containing an Rh antigen is said to be Rh positive (Rh+); blood lacking the antigen is said to be Rh negative (Rh–) . Unlike the case for the ABO antigens, the production of Rh antibody requires prior exposure to the antigen, such as would occur in an Rh– pregnant woman carrying a fetus that was Rh+.