In the last article we looked at how the immune system responds to a pathogen which is encountered for the first time. This is the primary response. However, if the same pathogen enters the body again in the future, the immune system is able to deal with it more quickly because it is prepared. In this article we will look at the differences between these two responses, and how immunity can be gained.
The primary response
When a pathogen breaks through the body’s defences for the first time, the foreign antigens trigger the immune response. The primary response is quite slow because it takes a long time to produce enough B cells to produce enough antibodies to destroy the pathogen. During that time, the infected person will become ill and have symptoms of the disease. But remember that during clonal selection and clonal expansion, T memory cells and B memory cells are produced. These memory cells remain in the blood for a long time and have the ability to ‘remember’ the antigen (T memory cells) and antibodies (B memory cells). So the person is now immune should the pathogen be encountered again.
The secondary response
If the same pathogen manages to get into the body again (sneaky), the memory cells are ready for action straight away. The antigen is recognised, and clonal selection happens very quickly. The T memory cells divide and differentiate into T killer cells specific for the antigen. The B memory cells divide and differentiate into plasma cells which rapidly produce complementary antibodies to the antigen. The secondary response is also stronger than the primary response, meaning that more antibodies are produced. The pathogen will often be destroyed before the infected person has begun to show symptoms of the disease.
Active, passive, natural, and artificial immunity
Immunity can be gained in a few different ways depending on what the immune system is exposed to and where it came from.
Active immunity is when the body produces its own antibodies after exposure to an antigen, and goes through the process of producing T memory cells and B memory cells. Therefore it is slow but provides long term protection. There are two ways to gain active immunity:
- Natural active immunity – by being exposed to the antigen by catching the disease.
- Artificial active immunity – by being exposed to the antigen through a vaccination (often a dead or modified form of the pathogen). We will cover vaccines in more detail in a separate article.
Passive immunity skips the majority of the immune response by introducing antibodies to the body. Therefore it provides immediate protection, but it will only be short term because memory cells are not produced and antibodies are broken down in the blood quite quickly. There are two ways to gain passive immunity:
- Natural passive immunity – when a baby receives antibodies from its mother through the placenta and breastmilk.
- Artificial passive immunity – when someone is injected with antibodies from another source e.g. another person.
Summary
- The primary response (in response to first infection) is slow and the person will show symptoms. Memory cells are produced.
- The secondary response is fast and stronger than the primary response because the memory cells can divide quickly to produce antibodies. The person will likely not show symptoms.
- Active immunity (which can be natural or artificial) is when the body produces its own antibodies and memory cells. It is slow but gives long term protection.
- Passive immunity (which can also be natural or artificial) is when antibodies are introduced to the body from another person. It is immediate but gives short term protection.
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