The aftermath of the immune system activation: Memory lymphocytes and vaccines

Today we are continuing our journey towards understanding the immune system. Last week, we finally won, and the pathogen was destroyed. However, the immune system is not done: we first need to calm it down to prevent potential damage, then it will repair the damage the pathogen has done, and finally, it will make sure that the next time we encounter this pathogen, we will be ready to fight.

Even if the pathogen has been cleared, the immune system is still working actively: it is searching for more antigens and infected cells so that it can destroy them. However, none are left. Further, many of the cytokines produced by the immune system are actively contributing to the disease symptoms. For instance, interleukin 6 is a cytokine that reaches the brain and activate a fever. In order for the body to get better, the immune system has to be deactivated, and this is done thanks to another subset of CD4 T lymphocytes: the regulatory T cells, or Treg cells. Treg cells are here to restore the balance: if the immune system is activated too strongly or for too long, it causes more harm than good, hence Treg cells dampens it. It is mainly responsible for the inactivation of proliferation and multiplication of lymphocytes, and can also completely inhibit them. It is done mainly through a cytokine called interleukin-10 (IL-10). When the immune system is inactivated, it has now a new purpose: repair. The pathogen has caused a lot of damage by infecting cells and fighting off the immune system, and as a results there is a lot of dead cell debris and « holes » in the tissues in place of infected cells. Many immune cells will be involved in the debris cleaning, particularly macrophages, and for the tissue repair to happen, many lymphocytes will produce cytokines to accelerate cell multiplication and replace the dead cells. We call this the immune system Type 2, which is focused solely on repair [source / source / source / source].

Now that the body is clean and healthy, it is time for the immune system to regroup: They do not want this fight to happen again. To prevent this, they will immortalize a small subset of activated lymphocytes, and call them memory lymphocytes. An example are the memory B cells. These will continuously produce antibodies against the pathogen they fought. As a result, our blood will have many of these antibodies, and if we encounter the pathogen again, they can already act without the adaptive immune system being active. Memory T cells also exist, but in many forms. Effector memory T cells will roam around the body, and as soon as they see a cell infected with the pathogen they know, they will kill it. Central memory T cells are located in lymph nodes and have very high proliferation capability, allowing it to produce many T cells very fast in case of infection. Regardless of the type of memory lymphocyte, the effect is the same: if the pathogen is encountered again, the immune system is activated faster, it is more efficient, and it already has the arsenal of the adaptive immune system, so hopefully we would not need to activate it [source / source / source / source / source / source].

The presence of memory lymphocyte makes the body ready for any disease that it already encountered, which prevents a lot of damages that are due to the immune system. So a question arose: can we prepare the body for diseases ourselves? Simply put, yes we can: if the body encounters specific antigens, it can prepare its defenses without going into a full on immune activation. This is the concept of vaccines. While I could talk about vaccines for a very long time, today I will simply answer some of the most asked questions about vaccines.

1. What is inside a vaccine?

There are many type of vaccines depending on the virus or bacteria that is targeted, but in general it is composed of two main ingredients. First, the active ingredient is the part you want to be protected against. It can either be the full pathogen that has been killed, or simply a part of its structure that contain the antibody. The amount of the active ingredient is extremely small, and it never contains a pathogen that can cause a disease. We simply put the antigens in the vaccine so that we can create memory lymphocytes. However, a small amount of dead pathogen will not activate the adaptive immune system, since the innate immune system can easily get rid of it, and thus, no memory lymphocytes will be formed. Therefore we need the second part of the vaccine: the adjuvant. The adjuvant is here to force the immune system to be activated. It simply stimulates it long enough for lymphocytes to receive the antigens and become memory cells. Any other components are either to help you fight the pathogen, or inhibit the pathogen. For example, the presence of formaldehyde is to inactivate the virus so that it does not cause a disease. The amount of formaldehyde is so low that it is impossible to be dangerous for humans [source / source].

2. Can vaccine cause harm?

Like any medication, vaccines can also cause problems. There are side effects, such as headache or fever, all associated with the activation of the immune system. Some cases have shown more severe problems, including bleeding or violent gastrointestinal problems. These cases are very rare, and unfortunately we do not know what is their cause, as it might not even be due to the vaccine itself, but another unrelated diseases. There are instances where vaccines can be very dangerous. If you take a drug regularly for example, it might have interaction effects. Some viruses also work in cooperation to infect humans, thus if you are sick it is not advisable to get a vaccine as the presence of a viral antigen can cause problems. Lastly, people with a dampened immune system cannot get vaccinated, considering that they cannot have a proper immune response. It is the case if you are undergoing cancer treatment, hormone treatment for sex reassignment, or suffer from diseases like AIDS. Since you are activating your immune system, you are bound to have side effects, but the benefits of vaccines greatly outweigh the risks. Many diseases you are vaccinated against are deadly, and vaccination is the only method that will save you. Still, if you feel concerned about potential side effects, or if you have family members that had problems with vaccines in the past, you have to talk about it to your doctor, who will decide if it is safe for you to get vaccinated [source].

3. Why get vaccinated?

We get vaccinated for two reasons: first, to make sure you can fight off the disease you are vaccinated against. But more importantly, vaccination contribute to the herd immunity. In the last question, I mentioned that some people could not get safely vaccinated, and as such they are at risk of developing these diseases. To protect them, we have to get vaccinated. Viruses mostly work by transmission from human to human. If most people are vaccinated, then a virus cannot enter the population and infect non-vaccinated people. However, if people do not get vaccinated, then the virus can enter the population and get transmitted to non-vaccinated people. Nowadays, we use vaccines to protect other people more than to protect ourselves [source].

4. Do vaccines cause autism?

No. It has been disproved by thousands of papers. There is no link between vaccine and autism, but it is interesting to understand where the idea came from. A scientist named Andrew Wakefield published in 1998 that the MMR vaccine was causing autism. This article sparked outrage in the public, making people wary of vaccination. However, no one in the scientific industry was able to reproduce Wakefield’s experiments and get the same results. That was because he fabricated the results. He modified his data to get what he wanted. It was also shown that he published this article for financial gain: he said the MMR vaccine was bad, but the one he created was good and people should buy it. Lastly, it was discovered that he performed unethical experiments on mice and humans. His medical license was revoked and the article retracted, but the seed was planted: people were suspicious of vaccination despite the huge amount of literature proving that no problem existed. Finally, this controversy also show the bias that runs in our society: people would rather have their children die than have autism. Autism is seen as a horrible thing, when autistic people live very well in our society. The lack of education on mental disorders vilify them [source].

5. Are vaccines safe?

I will answer this question by paraphrasing my old immunology teacher, who explained it best in my opinion: It depends on your definition of safe. If safe means free of harm then no, vaccines are not safe. But neither is taking Advil, or get an operation. There will be side effects but the benefit of being vaccinated greatly outweigh the risks. However, if safe means that it protects you from harm (like a seatbelt in a car), then vaccines are the safest medical procedures that exists nowadays. Vaccines are the only way you can protect yourself from dangerous diseases, many of which we do not have a treatment for.

This article was very important to me, especially considering what is happening nowadays: the measles outbreak was 100% preventable by vaccination, and the event proves that the anti-vaccination movement is extremely dangerous. We need to educate people more on vaccine to prevent other children from suffering.