Video Transcript
The following information is obtained from a sample of blood taken from a
patient. Antigens present on red blood cells: none. Antibodies present: anti-A and anti-B. What blood group is the patient most likely to be?
The question asks us to identify the blood group from a hypothetical sample of
blood. We are given two clues to help us with our task. First, the red blood cells in the sample have no antigens. And second, both anti-A and anti-B antibodies are present. Let’s have a brief review of blood groups so we can use these clues to find the
correct answer.
An individual’s blood group is determined by the combination of alleles, or
variations of genes, present in their genotype. Some of these alleles, known as IA, IB, and IO, control antigens that may be found on
the surface of our red blood cells. You may have learned that an antigen is a substance that triggers an immune
response. An individual with two IA alleles, one from each biological parent, will produce A
antigens on their red blood cells and be a member of blood group A. An individual with two IB alleles will produce B antigens on their blood cells and be
in blood group B.
Now, pause the video and see if you can figure out what type of alleles and antigens
would be present in someone with blood group AB.
Hopefully, you deduced that an individual in blood group AB would have one IA allele
and one IB allele. The more challenging aspect of this question is that the IA and IB alleles are
codominant, meaning that IAIB genotypes present both A and B antigens. The third allele, IO, doesn’t produce A or B antigens. So, someone with two IO alleles won’t have A or B antigens on their blood cells and
will be a member of blood group O. However, if an IA allele or an IB allele is inherited along with an IO allele, the
individual will have blood cells that present either A or B antigens and be in blood
group A or B, respectively.
Antibodies are proteins produced by the immune system in response to antigens. Each antibody can bind to just one specific type of antigen, and this binding
initiates immune processes to destroy the specific antigen and the cell to which it
is attached. The different blood types produce different antibodies against the A and B
antigens. For example, members of group A produce antibodies against B antigens, called anti-B
antibodies, while members of group B produce anti-A antibodies. And members of group O produce both anti-A and anti-B antibodies.
Can you figure out what type of antibodies a person in blood group AB would have? Okay, that was sort of a trick question, because this individual would produce
neither anti-A nor anti-B antibodies. If they did, their immune system would attack their own blood cells. This illustrates why a patient receiving a blood transfusion must be carefully
matched with a compatible blood donor. If an incompatible blood type is given, the recipient’s antibodies will recognize the
antigens on the foreign blood cells and set off an immune response. This can cause symptoms such as fever, low back pain, and shortness of breath. Interestingly, one of the first symptoms experienced by patients given the wrong
blood type is a feeling of impending doom.
Now that we have reviewed blood types and their associated antigens and antibodies,
let’s look back at our two clues. The patient in question has no antigens on their blood cells. They possess both anti-A and anti-B antibodies, meaning their body is primed to
initiate an immune response against any blood cells that do present A or B
antigens. Both of these clues are consistent with group O blood. So, the patient is most likely to be a member of blood group O.