Sue has ben diagnosed with systemic lupus erythematosus (SLE). She recognizes that this is one of the disorders that is referred to as an autoimmune disorder. What could you tell her that describes these diseases?
SLE is a systemic autoimmune disease. Autoimmune diseases are diseases wherein your own immune cells attack your own normal cells. Immune cells are an integral part of our body. They keep us safe. When a bacteria or a virus enters our system, the immune cells help in neutralizing these foreign organisms. For this reason, immune cells (T cells and B cells, among others) are very important for every human. An insufficiency of these cells is very harmful and can be fatal as in immunocompromised patients suffering from, for instance, severe combined immunodeficiency or from AIDS.
This means that we need a certain amount of all our immune cells for our body to be able to fight of bacteria and viruses on its own. It also helps in healing wounds, and others. All in all, immune cells are our protector. As protectors, they do not attack our own cells - this is what we call self-tolerance, they can tolerate normal cells, but would kill foreign cells.
However, in the case of autoimmune diseases, these protectors, or at the very least a subset of them, turn against us. An appropriate immune response is launched to the hosts, our own, cells and tissues during autoimmunity, and the self-tolerance property is dropped. There are a lot of reasons to develop autoimmunity. Usually, there would be genetic causes, although it's possible that it is because of a reaction to drug-usage, or environmental agents. Nonetheless, the effect would be to make the immune cells overly active to the point that they start to fail tolerating normals cells - thus attack us, the host.
Cures for autoimmune diseases usually involve immunosuppression. Since the patient's immune cells are the ones attacking him, we need to suppress these cells' activities until autoimmunity is cured. Of course, since these are our protectors, immunosuppression presents the risk of being attacked by opportunistic pathogens, those that takes advantage of the suppression of our immune cells.
In essence, in SLE, or autoimmune diseases in general, what happens is that our defense mechanism's, the immune system's, cells fail to do their job, by failing to tolerate self-cells.