Your immune system is made up of white blood cells and is the army of your body, it is in charge of keeping the invaders out. Invaders like viruses and bacteria. To do that, it produces molecules called cytokines which are the communication molecules of the immune system. Some cytokines are called Interleukins. Interleukin 10 is an inhibitory cytokine, meaning it tamps down the immune response. Interleukin 12, on the other hand is a pro-inflammatory cytokine, meaning it activates the immune system cells to divide and attack. There are many cytokines and they are produced by the cells of the immune system, such as T cells, B cells, Dendritic cells, Macrophages, Neutrophils and more (all white blood cells.)
The absolutely AMAZING thing about the immune system is that our immune cells can rearrange segments of their DNA to produce millions and millions of different molecules (called T cell receptors and Antibodies) which can recognize and bind to foreign antigens and pathogens. This gives us the ability to recognize and destroy millions of pathogenic things such as bacteria, viruses, parasites, fungus, molds and allergy causing molecules. Think about how many disease causing organisms there are, millions of bacteria, viruses, parasites, fungi, molds, poisons. The immune system can form T cell receptors and Antibodies to recognize all of these and at least give a very valiant effort to get rid of them!
So the T cells and B cells and Dendritic cells, and macrophages and more kinds of immune cells can make molecules that will recognize foreign pathogens and foreign molecules. Then these cells will engulf the pathogen or allergy molecule and digest it or destroy it, thereby getting rid of the pathogen or molecule.
However, as AMAZING and miraculous as our immune system is, if it mistakenly turns on ourself, it can wreak havoc on us. This is called AUTOIMMUNITY. When our immune system erroneously thinks that our joints are an invader and starts destroying them, then we get what is known as Rheumatoid Arthritis. Our immune system can start attacking our own pancreas, in which case we’ll get Autoimmune Diabetes. So when the immune system reacts against our own body, that is autoimmunity. All autoimmune diseases are a result of this. Steroids suppress the immune system, suppress the white blood cells from attacking our own organs, that is why steroids are prescribed for autoimmune diseases.
Same with life threatening allergies, for example a peanut allergy. The immune system goes out all guns blazing against something that is totally harmless like a peanut or other foods or even pets. Again steroids are used to suppress the immune system and for immediate treatment of allergies, epinephrine aka adrenalin is used to relax airways and tighten blood vessels.
(Just an amazing fact: Neutrophils, which are a part of the innate immune system, if they fail to phagocytize bacteria, they “spit out” their DNA and trap bacteria in these DNA nets, where the bacteria die.These Kamikaze neutrophils die as well!)
There are markers of inflammation which can be measured by blood tests. One such marker is C Reactive protein, another is increased levels of certain cytokines (such as IL12) another is simply white blood cell count. If any of the aforementioned markers are increased, inflammation is increased.
So now, what people are finding with depression and bipolar d/o, is that inflammation is increased, and when people are in remission from depression and or bipolar d/o, then inflammation and its markers are decreased.
So what does that mean? Well it could mean that our white blood cells are reacting to something perhaps in the brain and that reaction causes depression to happen. It could mean the cytokines that increase when inflammation happens are causing mental illness. It could mean a plethora of things. Hopefully, scientists will find out what soon and discover a treatment for this cause.
There’s also this article “Is Inflammation What’s causing Your Depression?” http://www.prevention.com/health/health-concerns/link-between-inflammation-pain-and-depression
Another article: “Mood Disorders Linked to Inflammation.” http://www.healthline.com/health-news/mental-mood-disorders-tied-to-autoimmune-diseases-infection-061213