immune disorders
Overview of the Immune System
Plan of Action
A successful immune response to invaders requires Recognition Activation and mobilization Regulation Resolution To be able to destroy invaders, the immune system must first recognize them. That is, the immune system must be able to distinguish what is nonself (foreign) from what is self. The immune system can make this distinction because all cells have identification molecules (antigens) on their surface. Microorganisms are recognized because the identification molecules on their surface are foreign. In people, the most important self-identification molecules are called Human leukocyte antigens (HLA), or the major histocompatibility complex (MHC) HLA molecules are called antigens because if transplanted, as in a kidney or skin graft, they can provoke an immune response in another person (normally, they do not provoke an immune response in the person who has them). Each person has an almost unique combination of HLAs. Each person’s immune system normally recognizes this unique combination as self. A cell with molecules on its surface that are not identical to those on the body’s own cells is identified as being foreign. The immune system then attacks that cell. Such a cell may be a cell from transplanted tissue or one of the body’s cells that has been infected by an invading microorganism or altered by cancer. (HLA molecules are what doctors try to match when a person needs an organ transplant.) Some white blood cells—B cells (B lymphocytes)—can recognize invaders directly. But others—T cells (T lymphocytes)—need help from cells called antigen-presenting cells: Antigen-presenting cells ingest an invader and break it into fragments. The antigen-presenting cell then combines antigen fragments from the invader with the cell's own HLA molecules. The combination of antigen fragments and HLA molecules is moved to the cell’s surface. A T cell with a matching receptor on its surface can attach to part of the HLA molecule presenting the antigen fragment, as a key fits into a lock. The T cell is then activated and begins fighting the invaders that have that antigen. T cells are part of the immune surveillance system. They travel through the bloodstream and lymphatic system. When they reach the lymph nodes or another secondary lymphoid organ, they look for foreign substances (antigens) in the body. However, before they can fully recognize and respond to a foreign antigen, the antigen must be processed and presented to the T cell by another white blood cell, called an antigen-presenting cell. Antigen-presenting cells consist of dendritic cells (which are the most effective), macrophages, and B cells. White blood cells are activated when they recognize invaders. For example, when the antigen-presenting cell presents antigen fragments bound to HLA to a T cell, the T cell attaches to the fragments and is activated. B cells can be activated directly by invaders. Once activated, white blood cells ingest or kill the invader or do both. Usually, more than one type of white blood cell is needed to kill an invader. Immune cells, such as macrophages and activated T cells, release substances that attract other immune cells to the trouble spot, thus mobilizing defenses. The invader itself may release substances that attract immune cells. The immune response must be regulated to prevent extensive damage to the body, as occurs in Autoimmune Disorders . Regulatory (suppressor) T cells help control the response by secreting cytokines (chemical messengers of the immune system) that inhibit immune responses. These cells prevent the immune response from continuing indefinitely. Resolution involves confining the invader and eliminating it from the body. After the invader is eliminated, most white blood cells self-destruct and are ingested. Those that are spared are called memory cells. The body retains memory cells, which are part of acquired immunity, to remember specific invaders and respond more vigorously to them at the next encounter. 