The immune system of a multicellular organism acts 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).The immune system is based on immune cells called leukocytes (or white blood cells) that are produced by stem cells in the bone marrow. The immune system can be divided into two parts. Many species, including mammals, have the following type:
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 signal the cell to "commit suicide", thus killing that cell and any viruses it is in the process of creating.
