Psoriasis is an inflammatory condition the cause of which is unknown at this time. There is certainly a genetic component to this condition and work over the last decade or so has shown that it is an immunological disease. It has long been known there is inflammation within the plaques of psoriasis.
A chance observation that an immunosuppressive drug called cyclosporine used to prevent rejection in individuals with organ transplants cleared psoriasis. This showed that white blood cells called lymphocytes were important in the production of psoriasis.
More recently a huge amount of work has defined many of the immunological events involved in psoriasis. Interestingly, the immune response may have strong similarities to that which occurs in patients with rheumatoid arthritis, Crohn’s disease and possibly multiple sclerosis.
There is an immune activation of white cells called lymphocytes. The type of lymphocytes called T-cell lymphocytes are activated and in turn produce chemical mediators known as cytokines. These cause the cells within the epidermis to grow quickly. Lymphocytes are part of the defense system and are in a sense the paratroopers of the immune system which attack a variety of antigens such as viruses. The skin is very active in the immunological organ which explains why we develop rashes as a result of allergic reactions to multiple substances including medications.
Part of the life history of a T-lymphocyte is that it is produced in the bone marrow and travels through lymph nodes and makes a journey through the skin – particularly the epidermis. An antigen is something such as a protein that we have an immune response to. Before this can happen there are specific cells whose function are to present antigens to lymphocytes and some of them live within the epidermis.
Once a lymphocyte has been introduced to an antigen it becomes activated and it becomes a specialist against a specific target. The antigen or the target for the immunologic attack in psoriasis is not known but it is clear that activated lymphocytes that are an important part of the process of making psoriasis. The interaction between the lymphocytes and the cells that introduce the antigen and the cytokines that are produced is very complicated.
Cytokines are chemicals such as interferon, interleukin or tumor necrosis factors. Luckily for us these activated lymphocytes will stay on the job of attacking something within the epidermis and do not go off to other parts of the body to cause inflammation. They do not prevent other lymphocytes from being available to fight colds and other infections.
The discovery of the interactions between these cells and the specific receptors on the cell urfaces as well as the chemicals that they release has given scientists an opportunity to develop drugs that block this cascade of activity at different points.