The immune system of a multicellular organism acs as a defense against pathogens, such as viruses, bacteria, parasites, and some poisons. There are several variations of immune systems throughout species, and sometimes more than one immune system within the same organism (for example, the human brain has its own immune system that is separate from the "normal" one). Many species, such as mammals, use the following "version".
- The humoral immune system, which acts against bacteria and viruses in the body liquids (such as blood). Its primary means of action are immunoglobulins, also called antibodies, which are produced by B cells (B means they develop in the bone marrow).
- The cellular immune system, which takes care of other cells that are infected by viruses. This is done by T cells, also called T lymphocytes (T means they develop in the thymus). There are two major types of T cells:
- Cytotoxic T cells (TC cells) recognize infected cells by using T-cell receptors to probe the surface of other cells. If they recognize an infected cell, they can kill the cell and all the viruses within.
- Helper T cells (TH cells) interact with macrophages (which ingest dangeous material), and also produce cytokines (interleukins) that induce the proliferation of B and T cells.
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