What is in blood?
Blood carries many things round the body. It consists of a liquid called plasma, in which various things are transported. These include red blood cells (erythrocytes), white blood cells, and platelets (which make the blood clot).We had a slide showing human blood on the microscope. The magnification was 800x, whereas the photo below shows 1,000 x. Red blood cells were easily visible, and here and there we saw irregular-shaped cells stained a different colour, which were white blood cells.
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| Blood smear under 1000x magnification. |
Blood types
The main blood types in the UK are A, B, AB and O. In addition there is Rhesus factor type, which is positive (+) or negative (-).Red blood cells carry substances on their surface called antigens. The blood types are named after the types of antigen on the red blood cells.
Antigen A and antigen B are two major blood group antigens. Blood type A has antigen A on the red blood cells, and blood type B has antigen B on the red blood cells. Blood type O does not have either antigen A or antigen B present. Blood type AB has both antigen A and antigen B on the red blood cells.
An antigen is something which can trigger an immune response if the body detects it as foreign. The immune system produces antibodies which attack the antigen. Normally this wouldn't be a problem, because your body does not consider your own red blood cells to be foreign; it recognises it as part of your own body. However, it becomes critical when someone is receiving a blood transfusion - getting it wrong can lead to severe illness or death. The reason this can happen is because blood can also contain antibodies which attack antigens A or B. These antibodies are naturally occurring - they do not need to be triggered by exposure to the antigen, unlike, say, antibodies to viruses. If you are blood group A, your blood plasma can contain antibodies which attack the antigens in blood group B. These are 'anti-B antibodies'. Similarly, if you are blood group B, your plasma may contain anti-A antibodies.
The other major blood group is the Rhesus factor. The Rhesus protein is either on your red blood cells, or not. If you have it, you are Rhesus Positive, and if you don't, you are Rhesus Negative. Normally this is just shortened to + or -, so if you are blood group O and Rhesus positive, you are said to be blood group O + .
What happens if the wrong blood is used?
If two incompatible blood types are mixed, antibodies in the plasma from one donor attack the antigens on the surface of the red blood cells from the other. The red blood cells clump together; this is known as agglutination. This clumping can cause problems with blood flow around the body. In addition, the red blood cells under attack from antibodies can start to break down, and the haemoglobin they contain is toxic when not safely contained in a cell.The safest blood to give in a transfusion is a person's own type, but if that's not available, you can always give type O- because it doesn't contain any of the A, B or Rhesus antigens which the recipient might have antibodies for. Someone with type O- blood is known as a 'universal donor' because they can give blood to anybody.
Someone with type AB blood has both antigen A and antigen B on their own red blood cells, so their immune system doesn't produce anti-A or anti-B antibodies. If they're AB+ too then it's even better for them - they don't produce anti-Rhesus antibodies either. These people can receive blood from anybody - their immune systems will not attack donated cells with antigen A, antigen B or Rhesus factor. They are known as universal recipients.
Testing for blood type
The agglutination reaction is easily visible and is used as a quick test of blood types. A drop of blood is mixed with anti-A serum, which acts like anti-A antibodies, anti-B serum, and anti-Rhesus factor serum.Family Mystery
We used synthetic blood to practise identifying blood types .Our blood samples were from:
Mary - mother of baby
Matthew - newborn baby
John
David
By mixing drops of blood with anti-A, anti-B and anti-Rhesus serums, we were able to establish the following:
| Blood Sample | Agglutination with: | Blood group: | ||
|---|---|---|---|---|
| Anti-A | Anti-B | Anti-Rhesus | ||
| Mary | No | No | No | O - |
| Matthew | No | Yes | Yes | B+ |
| John | Yes | No | Yes | A+ |
| David | Yes | Yes | Yes | AB+ |
Note that it's good practice to write down your observations - what you actually see happening - before you write down conclusions. In the table above, our observations are whether agglutination occurred or not, for each anti-serum. Our conclusions are what that tells us about the blood group, which is the final column. Breaking it down like this makes it easier to check your work, and for others to follow it.
Inheritance
This section is aimed at the children working at GCSE Higher level, but the younger ones may find it interesting too.
Blood groups are determined by your genes, and the main blood groups are a good example of how dominance works. You inherit one gene for your ABO blood group from each parent. Alleles are different versions of a gene, and the alleles here are A, B and O. A is dominant to O, so if you have one A and one O gene (your genotype is AO), then your blood group will be A.
B is also dominant to O, so someone with genes BO will have blood group B.
What this really means is that you only need one gene for the A antigen in order to produce the A antigen, and the same for the B antigen.
A and B are co-dominant or incompletely dominant over each other, so if you have one each of genes for A and B, your blood group will be AB.
Your genotype is the genes you have, and your phenotype is how you turn out. If your phenotype is blood group A, you could have genotype AA or AO, because A will 'win' and will produce the antigen even if you have only one copy of the gene for it. If your phenotype is blood group B, you could have genotype BB or BO; you produce antigen B even if you have only one gene for it. But if your blood group - your phenotype - is O, then we know your genotype must be OO, because if you were carrying a gene for A or B then your red blood cells would be producing some antigen A or antigen B.
Homozygous means you have two identical copies of a specific gene, while heterozygous means you have different copies. Someone who has genotype AA is homozygous for blood group ABO while someone with genotype AO would be heterozygous. Both would have blood group A, though.
Looking at our blood groups for the members of our mystery family, we can see that the mother is O, while the baby is B. John is A, so he cannot be the baby's father as this would not explain where baby Matthew got his gene for blood group B. On the other hand, David is blood group AB, so he can pass on genes for either A or B. We cannot say with this test whether David is the baby's father - only that it is possible that he is. However, we can say that John is not the baby's father.
Testing our own blood
Some of us tested our own blood type using blood-typing cards. These cards had a small amount of dried anti-A, anti-B and anti-Rhesus serum on them. A drop water was applied to each test field to make a solution of the anti-A, anti-B and anti-Rhesus (also known as anti-D for complex reasons!). We washed our hands, then swabbed the side of a fingertip with an alcohol wipe. Next, we used a single-use sterile lancet to prick our own fingers, and squeezed out a small drop of blood. A small amount of blood was applied to each test field, and we saw visible clumping on some areas. We didn't have anybody who saw no clumping in any test field - if we had, that would mean they were blood type O-.
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| Reuben's test card, showing clear agglutination for anti-A and anti-D. |
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| Reuben, Thaddeus and Angie are all A+, and Hugo is B+. |
Deductions about our own families
Alison knows that she is blood group O, but Reuben and Thaddeus's dad wasn't sure of his blood group. He thought he was group O. The boys tested themselves and found that they were both A, which means that their dad could be either blood group A, or AB.
Angie's mum knew that she was blood group O, but Angie's dad didn't know his group. He assumed he was O as well, because it is the most common group. However, Angie is group A and her sister Kate found out when giving blood that she is group B. We know Angie's dad is also the father of her sister, so that means Angie's dad can only be AB.
Antonia knows that she is O - , but her husband has never been tested. However, her four children have tested as A+, B+, A- and B+ . O and Rhesus negative are both recessive traits, so you need two genes for the O blood group, and two for Rhesus negative, to have this phenotype. This means that Antonia's children's father can only be AB+ group with genotype AB and Rhesus genotype +-, so carrying one allele of each. This means he is heterozygous for the Rhesus trait (carries copies of two different genes).
The most common blood type in the UK is O at 36%, followed by A at 30%. Type AB is relatively rare, at only 3% for AB+ and 1% for AB- , so having families with an AB member in our group is very lucky! You can find how common each blood type is on the Give Blood site.
Artificial blood?
Artificial blood of type O- (the 'Universal Donor' type) has been developed in a laboratory in Edinburgh, and trials in humans are due to start shortly. More about this from these links:NHS Blood and Transplant News - Trials of artificial blood to start within two years
Further Work
The Blood Typing Game - an award-winning game where you save patients by testing their blood type and choosing the best transfusions from a limited range of options.BBC Bitesize Characteristics and Inheritance - you'll need this if you're taking IGCSE Biology.
Here is a useful worksheet to test what you've learned about blood types and compatibility of blood donations.
Worksheet: blood type problems - another one, with plenty of short questions.
Nuffield Foundation: A Closer Look at Blood . Taking a sample of your own blood and preparing a slide.
Blood cells - labelled photographs and explanations of the cells found in blood.
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