The immune system is an extensive internal network that consists of cells, tissues and organs whose job it is to protect and defend the body from harmful invasion by foreign bodies such as viruses, bacteria, toxins, etc.
This system, sometimes referred to as the lymphatic organs, is spread throughout the body and each component has its specific job. Some act to recognize the foreign body, some manufacture the fighters, some distribute them, some regulate them, some produce the clean up teams, and some keep a record of the invasion so that a similar attack can be responded to effectively.
The bone marrow plays a major role in all of this. Here all the cells of the immune system originate. Some of these cells will remain in the bone marrow and mature into the so called B lymphocytes (literally, lymph-cells). Some of these B cells will produce the antibodies to respond to the invading foreign body. These antibodies are very specific proteins that recognize and lock onto a particular protein and by doing so trigger a response in other cells to move in, surround, destroy and remove the invading body. Some of these B cells will keep a record of the invasion and response for use in future.
Some cells that form in the bone marrow move out to the thymus gland where they mature into T lymphocytes. Like the B cells they come in a variety of types for a variety of jobs. Some trigger the B cells into action, some regulate the speed of reaction, some destroy, some clean-up.
These cells, carried throughout the body by the blood, are the front and centre operators of the system that scans, filters and fights for the well-being of the body.