Video Transcript
The method for determining a
person’s blood type by agglutination is outlined below. One, a sample of blood is taken
from the person who needs their blood type determined. Two, two large drops of blood are
placed on clean, sterile glass slides. Three, anti-A antibodies are mixed
in with one of the two drops of blood. Four, anti-B antibodies are mixed
in with the other drop of blood. What result, agglutination or no
agglutination, would you expect for type A blood mixed with anti-A antibodies?
This question provides us with a
method of determining someone’s blood type and asks us to predict the result when
blood type A is mixed with anti-A antibodies. To answer it, first, let’s look at
the four different blood types: A, B, AB, and O. The four blood types are an example
of a trait in humans that is controlled by a person’s genetics. There are three alleles, or
alternative versions of genes, that can combine to make up those blood types. There are A, B, and O alleles that
control blood type, written with a superscript over an I as shown in the second
column of this table.
If a person has two IA alleles or
an IA allele and an IO allele, they have blood type A. If a person has two IB alleles or
an IB allele and an IO allele, they have blood type B. If a person has an IA and an IB
allele, they have the AB blood type. Finally, if a person has two IO
alleles, they have the O blood type.
So, what does this mean? Well, the IA allele codes for the
production of type A antigens, which are proteins that can cause an immune response
and are presented on the surface of cells, in this case red blood cells. Similarly, the IB allele codes for
type B antigens. The IO allele doesn’t code for any
antigen. Therefore, A and AB blood types
have A antigens. B and AB blood types have B
antigens. And the O blood type has none.
Antibodies are proteins that are
produced by the body’s immune system in response to the presence of foreign
antigens, thus helping to fight potentially dangerous substances in your body. Blood may contain anti-A
antibodies, anti-B antibodies, both, or neither. Anti-A and anti-B antibodies can
respond to and attack A and B antigens, respectively.
Let’s go over an example. If someone has blood type A, they
would not produce anti-A antibodies because the type A antigen is not a foreign
substance to their body. If they did, they would be in
serious trouble because anti-A antibodies cause cells presenting type A antigens to
agglutinate, or clump together.
When an antigen is mixed with a
complementary antibody, they bind together. As antibodies have two binding
sites, they can bind together two red blood cells. Agglutination occurs as more and
more red blood cells are bound together in a clump.
On the other hand, someone with
blood type A would usually naturally produce anti-B antibodies. As antibodies are highly specific
to an antigen, anti-B antibodies cannot bind to type A antigens, so no agglutination
would occur. Now we know what response we would
expect if type A blood, which has A antigens, was mixed with anti-A antibodies. As anti-A antibodies agglutinate in
the presence of type A antigens, the correct answer is agglutination.
