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How Does the Immune Response Function?

Updated: Feb 28, 2021

Our bodies have a very sophisticated system within them to protect us from encounters by intruders that may harm us, such as viruses and bacteria. The immune system consists of specifically trained cells and also includes physical barriers such as the skin and mucous membranes that limit intruders to make it into the body. The immune cells are mainly found in the blood but patrol the whole body.

Two main systems: Innate and Adaptive

We have two main types of immune responses: one that is “quick and dirty”, and one that is more sophisticated but takes longer to establish. The first we call the innate immune response. As the name suggests, you are born with this and cells do not require previous training to work. These cells are able to immediately respond to a new intruder (virus, bacteria, cancer, etc.) without having seen it before. They form the first layer of protection that keeps us safe from our daily encounters with potentially harmful agents. However, in some situations, this is not enough to protect us from diseases.

The immune system does not just have a “quick and dirty” style, but also a more elegant and complex way of working. And, most importantly, the immune system remembers. The first time your body confronts a foreign agent, it will memorize how it looks, so the next time you face it, your immune system will react in a more effective and faster way to prevent the damage. The adaptive immune response is the part of the immune system that trains this memory response.

You are not born with it, but develop it over your lifetime. Every time you encounter something that is foreign to your body, this immune response will be triggered and will help you fight the disease. Your body needs about 10-14 days to get this adaptive response going. However, after fighting off an intruder, you develop a so-called memory for it. The next time you encounter it, the adaptive response can immediately take place, without having to learn.

The adaptive immune response is the part of the immune system that trains a memory response. This mechanism of the immune system is used in vaccines.

This is the mechanism we use in vaccines. By exposing the body to part of a virus, or an inactivated virus, we give the adaptive immune system the chance to learn about that intruder, without it causing you to get sick. Next time you encounter this intruder in real life, your adaptive system is ready to get into action right away, without needing the 10-14 days to get activated.

Important Cell Types for the Adaptive Immune Response

The immune system consists of many different cell types that all play essential roles, but a few are of particular importance when it comes to vaccine function.


There are a variety of different cells that are called antigen-presenting cells (APCs). An antigen is a fragment of the intruder or the foreign agent. For example, an antigen can be a piece of a protein that is part of a virus. The APCs use antigens as pieces of information to train and teach other cell types from the immune system what the source of that antigen looks like. This process is called antigen presentation.

T cells

Killer T cells respond to an antigen. Each cell has components within it that cut each protein in the cell up and ensures pieces of it are made visible for the T cells to see. This allows the T cells patrolling the body to check all cells and to ‘see’ what is happening inside of the cell. Once T cells recognize something that does not belong, they bind to the cell and kill the cell, hence the term killer cells. An example of a situation in which this would happen is in a virus-infected cell.

B cells

B cells have several functions in the immune response. Importantly, B cells can transform into plasma cells, the cells that produce antibodies. This is another essential aspect of the adaptive response, and antibodies are important products we aim to get from vaccination.


Antibodies are proteins that can attach to viruses or bacteria to mark them for destruction. They can also have a so-called neutralizing function. For instance, they can bind specifically to a structure of the virus that is required to enter your body’s cells. This will block this mechanism and prevent the virus from infecting cells.

In Conclusion

Various factors play together in a sophisticated immune response to get rid of any intruder. As some of the more important responses take a while to develop, a new virus will have time to spread in the body and cause harm to cells and organs. This results in disease and is the reason why vaccines are often needed to help prepare the body and prevent diseases from happening.

Contributed by: Text: Maartje Wouters, Illustration: Armando Andres Roca Suarez

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