Antibodies in our blood constantly search through the body, making sure that only
the proper molecules are present. This is essential, because they protect us from
invaders like bacteria and viruses. They can cause problems, however, in medical
treatment. For instance, if blood is given in a transfusion, it needs to be similar to
the normal blood, so that the antibodies are fooled into thinking that nothing is
amiss. By studying these similarities, researchers have discovered that blood comes
in several types, which define groups of people with compatible blood. The ABO
system defines one of the major types determining groups of people who can donate
blood to each other.
What's Your Type?
Proteins and lipids on the surfaces of red blood cells are covered with carbohydrate
chains, which form a protective coat around the cell. The ABO blood type is
determined by the type of sugars that are used to build these carbohydrates. The
carbohydrate is built around a core of 5-13 sugars, called the H-antigen, that ends in
a fucose sugar. For people with the O blood type, the story ends there. For the A and
B types, however, a specific glycosyltransferase, such as the one shown here from PDB entry
adds one more sugar to the end. For type A, this sugar is N-acetylgalactosamine,
and for type B, it is the slightly smaller sugar galactose. This
small difference, however, has a critical effect.
The immune system searches through its repertoire of antibodies and
gets rid of any that would attack the normal molecules found in the body. So, a
person with type A blood doesn't have any antibodies to bind to the type A sugars.
They do, however, have antibodies that attack type B carbohydrates, which will
attack any type B blood cells that are transfused. People with type O are great for
donating blood, since it will not cause problems with people with type A or type B
antibodies, but if they need a transfusion, their options are limited since they have
both of these types of antibodies in their own blood.
Binding Blood Carbohydrates
The blood type carbohydrates provide ready handles for attachment, and are
exploited by many organisms. A few examples are shown here. At the left is a lectin
from a seed (PDB entry
with four binding sites for carbohydrates (in all of
these pictures, the structure includes a few sugars from the longer natural
carbohydrate chain, shown here in green). The biological function of seed lectins is
not fully understood, but may be part of the defense of the plant. The function of the
two proteins on the right is more obvious--they are proteins from a bacterium and from a
virus, which are involved with the process of infection (PDB entries