Immunity: Grade 9 Understanding for Biology IGCSE 2.63B
The most complicated topic in the human transport topic is certainly immunisation. In a previous post, I said you should be able to answer the following two questions:
Why is it that the first time your body encounters measles virus, you suffer from the disease measles? Why will someone who has had measles as a baby (or been immunised against it) never contract the disease measles even though the virus might get into their body many subsequent times?
I thought in this post I should attempt to expand a little so as to provide answers to these two important questions. This understanding is quite complex for IGCSE but you cannot really see how immunity works unless you can work through each stage in the process.
Let’s pretend you are a new born baby and you get measles virus particles into your bloodstream from contact with an infected person. Remember viruses are not living organisms as they are not made of cells and have no metabolism. All they are is a tiny particle made of DNA (genetic material) surrounded by a protein coat.
Look up a picture showing the structure of a virus particle in Google. This one comes from science learn.org.nz
The “spikes” on the surface of the virus particle are proteins that are essential to allow the virus to get inside a host cell. But they can also act as antigens allowing the immune system to recognise the virus as a foreign object and so mount an immune response to it.
In the body there are hundreds of billions of white blood cells called B lymphocytes. Each B lymphocyte is able to divide by mitosis over and over again to form a clone of cells called plasma cells. These plasma cells secrete a type of protein called an antibody which has a shape specific to the shape of the antigen such that it can bind to the antigen and neutralise it. (Can you think of another example in the specification where the shape of a protein is essential to its function?)
Now here is the first key piece of information needed in understanding immunity. Each B lymphocyte is only able to produce an antibody molecule with one particular shape. So the reason you need hundreds of billions of lymphocytes is to be able to produce antibodies that have the correct shape to combat hundreds of billions of possible shaped antigens on a lifetime of pathogen exposure.
Go back to your newborn baby exposed to measles virus. There might be only a handful of B lymphocytes in the babies’ body that just happen to be able to produce a shape of antibody specific to antigens on the surface of the measles virus. Before any antibodies can be produced, the “correct” B lymphocyte has to come into contact with measles virus particles and be activated. It then has to divide many times by mitosis to form a clone of plasma cells and the plasma cells have to differentiate and start producing antibodies. This whole process is called the primary response (first exposure hence primary) and it may take up to 8 days before any antibodies start appearing in the babies’ blood. What are the measles virus particles doing all this while? Well they are infecting host cells, damaging them and causing disease. This is why the baby will suffer from the disease measles.
The second key piece of information for immunity is this: when the B lymphocyte that has been activated divides by mitosis to form a clone, not all the cells produced form antibody-producing plasma cells. About 25% of the clone just remain as lymphocytes and are called memory cells. This is because they are long-lived cells that account for immunological memory.
Let’s pretend the baby gets better from measles due to the antibodies produced in the primary response. What happens if years later, the child goes to school and meets measles virus again for a second time? You all know that the child won’t get the disease measles this time. This is because the immune response is different second time round – the secondary response. The secondary response to antigen is quicker (no 8 day delay), larger (more antibodies made) and lasts for longer. This is because in a secondary response there are not just a handful of B lymphocytes in the body capable of making antibodies to combat measles virus. There are now millions of memory cells left over from the primary response that can all immediately “leap into life” and start making antibodies. These antibodies will be produced so quickly and in such large numbers that the virus particles will be eliminated before they have time to cause harm and disease. No harm caused to host cells therefore no disease measles this time round!
Finally, you know that you can have immunity to measles without having had the disease. This is because everyone in the UK sitting GCSE exams this summer will have been immunised against measles virus as a baby. You were injected with antigens from the surface of measles virus particles when you were a baby. These antigens by themselves could not give you measles (why not?) but they did cause a primary response to occur and memory cells to measles antigens be formed. So now if you do encounter measles virus, your body will mount a secondary response and you won’t get the disease. #result
When answering questions on this topic in exams, candidates often think that it is the antibodies produced in the primary response that are left over to stop you getting measles later in life. Look at a graph showing primary and secondary responses to antigen such as the one below.
Antibodies are proteins and you can see they have a half-life in the blood of a few weeks. (The liver breaks down proteins in the blood as one of its many functions) So all the antibodies from a primary response will have been removed within a few months of the first exposure. Immunity can last a lifetime and this is because memory cells can survive as long as you do. Unlike antibodies they can hang around in your blood and lymph nodes for the rest of your life. If you live to be a hundred, you still won’t catch measles more than once.
This is a tricky topic so do please comment on this post if you have any questions. Work hard at revision – it will be worth it in the end….. (At least with Biology revision, it is fascinating stuff isn’t it?)
How does HIV affect the immune system? I looked at the markscheme for the question and it said invasion of lymphocytes. What does that mean?
HIV is a virus that can infect a particular type of white blood cell called a T helper cell. T helper cells are an example of a cell type called a lymphocyte. When the virus infects these cells, they cause the host cell to stop working and ultimately die. This loss of effective lymphocytes means the patient’s immune system no longer works and they develop the disease AIDS. Hope this helps!