When the immune system goes wrong

Today we will finally end our epic journey towards understanding the immune system. We have covered a lot on the subject, and we know that the body is amazing at protecting itself. Unfortunately, the immune system is not prefect, and there are many ways it can go wrong. Today we will review three ways where the immune system is not behaving properly: allergies, autoimmune diseases, and immunodeficiency.

Allergies, or when the body is desperately looking for problems when there are none.

The concept of allergies is simple: the body will react too strongly to a harmless antigen. In cases of allergies, antigens are called allergens. Allergens most often come from food or pollen, but virtually anything could cause an allergic reaction. The development of allergies is similar to any inflammatory reaction: an allergen enters the body and is picked up by dendritic cells to be presented to lymphocytes. Allergens causes the CD4 T cells to develop into a Th2 response, increasing antibody production against the allergens. This time however, a very specific type of antibody, called IgE, will be produced. These IgE will bind to a special type of white blood cell called the mast cells. These cells are loaded with large vesicles containing lots of cytokines. At the end of this phase, the mast cells will have many IgE and will be ready to act the next time the allergen comes. This phase is known as the sensitization. At this point, we have developed an allergy, but we haven’t suffered from it yet. If the allergen comes back, then it will be picked up by the mast cell, which will then unload all of its cytokines, resulting in an immune attack. The problem is, there is nothing to attack. Therefore, you end up with the allergic symptoms (itchiness, sneezing, coughing, etc…) but they usually resolve when the allergen is removed. However, if there is a consistent exposure to the allergen, then more damage will occur: cell death, irritation of the lungs, and many more will occur. Lastly, for still unknown reasons, some allergies are deadly. They lead to what is called an anaphylactic shock, which is due to the massive cytokine release all over the body, leading to a very strong immune response everywhere. Whereas allergies are usually limited to one part of the body, anaphylaxis is on the entire body, making it very dangerous. The most deadly effect is the constriction of heart and lung muscles, which prevent the person from breathing correctly, and the heart may stop. The blood pressure also drops significantly. Fortunately can be stopped if we act fast. The easiest way is using an EpiPen, which injects a hormone called epinephrine (also called adrenaline), which opposes every effects of anaphylaxis: it increases blood pressure, and relaxes muscles [source / source / source / source / source].

Allergies can be very dangerous, but unfortunately we still don’t know everything about them. While we understand how they work, we do not know why certain individual develop allergies while other don’t. We however have many hypotheses, and one of them is the hygiene hypothesis. During evolution, humans have evolved to clean themselves and their environment, which drastically reduced the incidence of diseases. Unfortunately, it also reduced exposure to those diseases. Exposure is what helps the immune system understand what is dangerous and what isn’t. Nowadays, the almost too-clean environment prevents a decent exposure, which makes the body more wary to everything. While this hypothesis is not proven, it is supported by the increase in allergies nowadays. However, the hygiene hypothesis cannot be the only explanation to allergies, and many think that there is a heavy genetic component. It would explain why certain countries are more at risk to allergies than other. In the end, we still need to study allergies to better treat people that suffer from it [source / source / source].

Autoimmune diseases, or the body attacking itself

I mentioned in the article on the innate immune system how essential the self vs. non-self discrimination was. A failure to discriminate both will result in autoimmune diseases. People suffering from these diseases will have their own immune system attacking specific parts of their body. Today I will talk about two common ones: diabetes type I and multiple sclerosis [source].

Type I diabetes is caused by an immune attack against the beta-cells in the pancreas. This results in a complete loss of insulin. Insulin is a hormone essential for glucose control. When you eat something containing carbohydrates, the body will break them down into glucose, and it will go into the blood. Insulin will then make sure that the cell will pick up the glucose to use it as energy. In type I diabetes, the absence of insulin prevents the cell from using glucose, causing hyperglycemia, which means too much glucose in the blood. It causes intense hunger and thirst, and dangerous weight loss. Unfortunately, there is no cure to type I diabetes, and we do not know why the disease occur. There is a treatment however, which is simply to inject insulin to allow the body to use glucose [source / source / source / source].

Multiple sclerosis is a more complicated disease that involves the neurons. People with multiple sclerosis have an immune attack against a part of the neurons called the myelin. It results in neurons being less efficient. It results in many symptoms such as vision or motor problems, balance issues, and more. Multiple sclerosis is unique to every individual, making it very difficult to treat. Worse, many cases worsen over time: for every immune attack, the symptoms get stronger and stronger. Unfortunately, there is no cure for this disease either, and treatment will only prevent the worsening of symptoms. The cause of multiple sclerosis is unclear, but there are evidence of a strong environmental cause such as viral infection or smoking [source / source / source].

Immunodeficiency, or the lack of the immune system

Like the name suggest, immunodeficiency is the absence of an immune system. More commonly, no lymphocytes are present or functional. These disorders are particularly dangerous because while they won’t kill you by themselves, any infection following it will. The two most common cases of immunodeficiency are AIDS and SCID.

AIDS, or Acquired ImmunoDeficiency Syndrome, is the advanced disease caused by the HIV virus. When a person is infected with HIV, the virus will enter CD4 T lymphocyte and prevent their normal function. This will lead to a dysfunctional immune response. If HIV persists and is not removed, some unknown mechanisms cause the complete stop of CD4 T lymphocyte formation, and the immune system is completely inhibited. Again, there are no cure, but if diagnosed soon, living with HIV/AIDS is very manageable. IF the HIV virus is found early on, it can be eliminated, preventing the development of AIDS. [source / source].

SCID, or severe combined immunodeficiency, is a disease affected newborn where they completely lack lymphocytes. Any disease or infection is now deadly. The only cure for SCID is a bone marrow transplant. This syndrome was made famous because of a boy named David Vetter, who suffered from SCID. No transplant was available for him, thus he lived in a bubble in a hospital to protect him from germs. He survived for 12 years, which is very high considering that newborn with SCID at the time died very early. This story influenced the movie Bubble boy [source / source].

This article marks the end of the story on the immune system. There is of course much more to study and understand, but this series is a good start. I hope that I made the topic interesting, and don’t worry, we will come back to the immune system very soon, when we go into microbiology.