Wikipedia: Immune system

The immune system of a multicellular organism acs as a defense against pathogens, such as viruses, bacteria, parasite?s, 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 immune system is based on immune cells called leukocyte?s (or white blood cells) that are produced by stem cells in the [bone marrow]?. The immune system can be divided into two parts :

The humoral immune system, which acts against bacteria and viruses in the body liquids (such as blood). Its primary means of action are immunoglobulin?s, also called antibodies, which are produced by [B cell]?s (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 cell]?s, also called [T lymphocyte]?s (T means they develop in the thymus?). There are two major types of T cells:
- [Cytotoxic T cell]?s (T_C cells) recognize infected cells by using [T-cell receptor]?s 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 cell]?s (T_H cells) interact with macrophage?s (which ingest dangeous material), and also produce cytokine?s (interleukin?s) that induce the proliferation? of B and T cells.