Blood is a complex, living tissue that contains many cell types and proteins. A transporter, regulator, and defender, blood courses through the body carrying out many important functions.
PROTEINS & BLOOD TYPES
Distinct molecules called agglutinogens (a type of antigen) are attached to the surface
of red blood cells. There are two different types of agglutinogens, type "A" and type
"B". Each type has different properties. The ABO blood type classification system uses
the presence or absence of these molecules to categorize blood into four types:
Another level of specificity is added to blood type by examining the presence or
absence of the Rh protein. Each blood type is either positive "+" (has the Rh protein) or
negative "-" (no Rh protein). For example, a person whose blood type is "A positive" (A
+), has both type A and Rh proteins on the surface of their red blood cells.
BLOOD TYPE IS GENETIC
The A and B antigen molecules on the surface of red blood cells are produced by two
different enzymes. These two enzymes are encoded by different versions, or alleles, of
the same gene: A and B.
The A and B alleles code for enzymes that produce the type A
and B antigens respectively. A third version of this gene, the O allele, codes for a
protein that is not functional and does not produce surface molecules. Two copies of
the gene are inherited, one from each parent. The possible combinations of alleles
produce blood types in the following way:
WHEN BLOOD TYPES MIX
Blood plasma is packed with proteins called antibodies. The body produces a wide
variety of antibodies that will recognize and attack foreign molecules that may enter
from the outside world. A person's plasma does not contain any antibodies that will
bind to molecules that are part of his or her own body.
When conducting a blood transfusion, it is important to carefully match the donor and
recipient blood types. If the donor blood cells have surface molecules that are different
from those of the recipient, antibodies in the recipient's blood recognize the donor
blood as foreign. This triggers an immune response resulting in blood clotting. If the
donor blood cells have surface molecules that are the same as those of the recipient,
the recipient's body will not see them as foreign and will not mount an immune
There are two special blood types when it comes to blood transfusions. People with
type O blood are universal donors because there are no molecules on the surface of
the red blood cells that can trigger an immune response. People with type AB blood
are universal recipients because they do not have any antibodies that will recognize
type A or B surface molecules.
Note: Blood cells are covered with a variety of surface molecules. For simplicity, only type A and B
surface molecules are shown here.