The immune system works to protect the body from infection. It monitors the body constantly and springs into action when it senses a foreign presence such as bacteria, a virus, or a fungus. In some situations, such as a cut or scrape, physical injury by itself is sufficient to set the immune system into action to protect against possible infection. The immune system is comprised of a specialized group of cells and organs. Among the blood cells, the white blood cells are active in the immune system.
Specialized white cells like T cells, B cells, and neutrophils (cells that make up pus) are important parts of the immune system. The immune system also has specialized locations in the body, including the lymph nodes and organs such as the spleen, thymus, tonsils, and appendix. The communication between these many cell types and organs is remarkable, and it occurs throughout the body through signals in the blood.
When the immune system is functioning properly, it guards against both external infections and internal damage. For example, the immune system watches for cells that could become cancer and tries to stop them. At times, however, the immune system is not watching closely enough and may miss a precancerous cell that later turns into cancer. At other times, though, the immune system may be watching too closely and decides that some part of the body—skin cells, or keratinocytes, in the case of psoriasis—is not healthy or does not belong and may become activated.