The immune system is the police of the body. The immune police cells are the white cells of the blood, and they patrol the blood and the lymph looking for troublemakers like bacteria, viruses, dead tissue or bruises. No matter which enemy is detected, the patrolling police cells use the same weapons to contain the bad guys as much as possible and to report the finding to the more sophisticated units. They do so first by causing inflammation, which translates into a) increased blood flow to the affected area to facilitate the arrival of more white cells; and b) pain in the area to alert us (and our brain!) that something is not right. Second, they try to deal with the threat by throwing chemical weapons like histamine or even by eating and digesting the bad guy to get it out of the way. This first general response is called innate response.
The role of the more sophisticated units (the adaptive response) is to keep a register of the usual suspects and design specific weapons (antibodies) for each one of them. Once the body has antibodies against a specific bug, the adaptive response can more efficiently identify and fight it. This is why we only get most diseases once. For example, people who have survived Ebola have antibodies against it, their body knows how to deal with it; that is why new patients are treated with blood from survivors, so they can benefit from those antibodies. This is also how vaccines work (read “Further Explanations” below for more information).
The key is how good the white cells are at distinguishing friend from foe. They learn to do so in the spleen and the bone marrow, which work as a police academy. If the cells do not prove that they are efficient are attacking the bad guys whilst respecting the good guys, they are not allowed into the blood and they are forced to commit suicide. Autoimmune diseases frequently start when this system fails and unprepared white blood cells are released into the blood.
The reason why that happens is not fully understood, but it is believed to be a mix of complex genetic and environmental factors. For example, a body might be genetically not very good at telling good bacteria from infectious bacteria. So after an infection it might create imprecise antibodies which will recognize and attack the bacteria that normally live in the gut. This can cause chronic inflammation, bad digestions, diarrhoea, and ultimately irreversible damage to the gut. That is very basically what we think happens in Crohn’s disease or ulcerative cholitis.
Similarly, type 1 diabetes happens when antibodies are created by mistake against insulin. Multiple sclerosis starts with antibodies against myelin (a protein which covers and protects some of our neurons. I don’t have space in this post to discuss these diseases which are very complex in themselves; I also don’t have space to talk about therapies for autoimmune diseases, but that will definitely be the subject of a future post. However, I hope you now have a better idea of the “what” and “why” of autoimmune diseases!
Inflammation. In spite of the fact that we constantly take anti-inflammatory drugs like aspirin or ibuprofen, inflammation is actually one the body´s most efficient weapons to protect us. When there is a threat, the white cells release substances in the blood in order to cause pain (the alarm that something is wrong!) and in order to increase the blood flow into the area- hence the swelling, the redness and the heat. The increased blood flow will make it easier for other white cells to get to the area and help and focus the efforts of the body to solve the problem.
Eating and digesting the bad guys: The technical name for that is phagocytosis. Some cells can engulf the bad guys or the cell debris they need to get rid of, which get trapped inside the cell in a wee “stomach”, where they get digested. This is a very clean way of getting rid of threats, as no toxic substances are poured onto the membranes of the body.
Histamine (and other chemical weapons): If for whatever reason the body can’t eat and digest the enemy, they go for other options like throwing chemical weapons at them, such as histamine and hydrogen peroxide, which are very toxic for the enemy (but also for our own body). Actually it is histamine release onto our membranes that causes hay fever. This, logically, happens mainly in areas of the body that are sensitive but exposed, around the respiratory system (nose, mouse and throat), in the eyes. And that is why antiallergy drugs are called antihistamines!
Antibody: It is a “Y” shaped protein which is produced by a subtype of white cells against a specific threat. It is the key weapon of the adaptive response. The tips of the “Y” recognize specifically their target and bind to it. The trunk of the “Y” interacts with other white cells and immune proteins, activating a very aggressive immune response.
Vaccine: Most vaccines are just a very weak or dead version of the virus or bacteria that causes an infectious disease. Exposure to that stimulates the immune system to create antibodies so that when the real virus or bacteria enters the body, it is prepared to defend itself very efficiently.
Cell suicide or apoptosis. Often the body needs to get rid of a cell, because it is old, or dysfunctional, or not needed anymore. That cell is going to receive the instruction to undergo apoptosis, so it is going to die in a very organised way so all the toxic substances that it contains remain inside the membrane and do not damage the cells around it. Think that each cell burns its food and needs to carefully treat and detoxify its residues before pouring them into the blood! If the dying cell just exploded all those untreated and very toxic residues would be uncontrollably released to the blood, causing huge problems and great pain. That is what happens for example when we have a limb that has been damaged by gangrene. The tissue is dying uncontrollably and has to be removed so it does not kill the neighbouring tissue.
Some cells learn to escape from those apoptotic signals and that is a frequent start point in cancer… but that is another story!