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Most higher organisms have some means of activating their immune systems to mount defenses against invading organisms. Immunocompetence is a term that refers to this process of activation. More specifically, it pertains to the way that the immune system recognizes molecules denoting a foreign object, called antigens, and creates appropriate responses to them. This process usually involves either a humoral or a cell-mediated response, which use different types of immune cells.
Humoral immunocompetence uses antibodies to recognize foreign particles. Antibodies exist throughout the human body, and contain regions that can bind to antigens, which are usually proteins that are not a part of the body's own cells. After binding an antigen, nearby or attached immune system cells, such as B-lymphocytes or plasma cells, can directly attack the invader. They may also release other chemical messages that cause other immune cells to come to the area to assist with the response.
Cell-mediated immunocompetence involves a somewhat different process. This form of response uses cytotoxic or helper T-lymphocytes. Each type of T-lymphocyte has proteins on its surface that can recognize cells that have entered a disease state, such as that due to a viral infection or cancer. Cytotoxic T-lymphocytes act to destroy the infected cell. Helper T-lymphocytes, on the other hand, release compounds that act to produce an immune response, increasing blood flow to the area, recruiting other immune cells, and causing inflammation.
Proper nutrition can be vital to an individual's ability to maintain immunocompetence. People that do not receive adequate amounts of vitamin D, for example, may become immune-compromised, and are much more likely to develop many types of medical conditions, from cancer to infectious diseases. Autoimmune diseases, where the body's immune system inappropriately attacks its own tissue, have the potential to develop in immune-incompetent people, as well.
Some external circumstances can compromise individuals' immunocompetence. The human immunodeficiency virus (HIV), selectively attacks and infects immune cells so that they cannot mount proper responses to foreign organisms, which can lead to death from conditions that would be relatively mild in healthy individuals. Transplant patients may take drugs to dampen the immune response, to prevent the immune system from attacking the foreign tissue placed into the body. People taking these types of medications can be characterized as immune-compromised, because their lymphocytes and other immune cells may not be capable of launching a full response after recognizing an antigen.