The immune system is a complex organisation of special cells, proteins, tissues and organs that defends against invading micro-organisms. Through a co-ordinated series of steps called the immune response, the immune system attacks organisms and substances that invade the body and cause disease.
The cells that are part of this defence system are white blood cells, called leukocytes. They come in two basic types, which together seek out and destroy the organisms or substances that cause disease.
Leukocytes are produced or stored in many locations throughout the body, including the thymus, the spleen and the bone marrow. For this reason, these are called the lymphoid organs. There are also clumps of lymphoid tissue throughout the body, primarily in the form of lymph nodes that house the leukocytes.
The leukocytes circulate throughout the body between the organs and nodes by means of the lymphatic vessels. They can also circulate through the blood vessels. In this way, the immune system works in a co-ordinated way to monitor the body for invading substances and micro-organisms that might cause problems.
The two basic types of leukocytes are:
1) Phagocytes: cells that chew up invading organisms.
2) Lymphocytes: cells that allow the body to remember and recognise previous invaders and help the body destroy them.
When an invading organism has been challenged by the B lymphocytes and destroyed, the antibodies produced continue to exist in the body, so that if the same antigen (invading organism) is presented to the immune system again, the antibodies are already there to do their job. That's why if you get sick with, for example, chickenpox, you are unlikely to get sick with it again. This is also the theory underpinning the development and use of vaccines: by artificially forcing the body to produce antibodies to the targeted invader, when the invader is presented to the immune system for real, the antibodies are already in the system, ready to combat the infection.
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