Immune System: Immune Response Steps Part 2: The Adaptive Immune System
Adaptive / Specific Immunity
The adaptive immune system is unique to vertebrates. It is a more targeted approach to dealing with infections. It is activated by the complement system of the innate immune system and so usually doesn't come into play until a few days after the initial onset of symptoms. The adaptive immune system allows the body to recognize and remember a specific antigen. If this antigen enters the body again, a quick, tailored immune response can quickly ward it off. Adaptive immunity is the reason that you can't catch many diseases more than once. There are two major types of adaptive immunity: cell-mediated and humoral immunity.
Cell-mediated immunity is particularly good for dealing with virus-infected cells, but it also plays a part in dealing with the bacteria that live inside cells. It functions by activating macrophages, natural killer cells, and antigen-specific cytotoxic T lymphocytes. It also regulates the release of cytokines, the proteins responsible for mediating the inflammatory response and communicating with other parts of the immune system. Macrophages swallow up antigens. Once they"ve processed the foreign matter, they display identifying markers for the antigen on their surface. This primes the T cells to recognize the antigen. The T-cells are produced in the bone marrow and matured in the thymus. They are essential to cell-mediated immunity. They are covered in receptors that allow them to sense the presence of an antigen. T-cells come in four main categories:
- Cytotoxic T-cells.
- Helper T cells.
- Suppressor T-cells.
- Memory-T cells.
The second type of adaptive immunity is humoral immunity. Humoral immunity is so called because it involves the fluids of the body (the humours), particularly the blood and lymph. In humoral immunity, B-cells are key. The B-cells, like the T-cells, are produced in the bone marrow. Again, like the T-cells, B-cells are covered in surface receptors that allow them to detect the presence of an antigen. However, unlike the T-cells, the B-cells do not, themselves, attack the pathogen. Rather, when they come into contact with an antigen, they withdraw to the lymphoid tissue and begin to divide rapidly. Most of these clonal cells then quickly become plasma cells. These plasma cells, in turn, begin to make antibodies to the antigen. Antibodies are y-shaped molecules with binding sites for a specific antigen. They bind to antigens and help the rest of the immune system identify them. Depending on where they bind to the antigen, they can sometimes also destroy it themselves. Those few B-cells that did not turn into plasma cells become the memory B-cells. Like the memory T-cells, the B-cells retain the ability to identify the antigen. They also continue to produce antibodies long after the infection is over. These antibodies enter the bloodstream and continuously move around the body. If they encounter the antigen, they quickly bind to it and activate the immune response.