The immune system is made up of a network of immune cells that are generated in the bone marrow from a very basic type of cell called a stem cell . From stem cells, many different types of immune cells are generated. The cells of the immune system circulate through the body in the blood or in a system of channels similar to blood vessels, called the lymph system , or congregate in special areas called the lymph nodes , which store immune cells. Lymph nodes are distributed throughout the body.
Some immune cells have general functions.
Macrophages and phagocytes patrol the body, eating dead cells, debris, viruses, and bacteria. Dendritic cells are more stationary and monitor the surrounding environment from one spot, like the skin. Other cells recognize and are activated by one single substance. When a lymphocyte recognizes its antigen and is activated, the lymphocyte makes many identical copies of itself, each recognizing the same antigen . T and B lymphocytes are called specific cells because they recognize only one substance while the other immune cells can recognize many different substances and are called non-specific cells .