The human body's own defence system is called the Immune system. It comprises many specialised tissues and cells such as blood, the bone marrow, the thymus (a gland located in the neck), the lymph nodes, the lymphatic tissue in large organs of the body such as the liver, the spleen and the gut. In the blood, it is the white blood cells or leukocytes, esp. the lymphocytes, that regulate the immune system. Control of immunity is also through the formation and secretion of many important substances such as the "lymphokines".
A key element of our immune system is its ability to distinguish between the human body’s own cells (referred to as ‘self’) and foreign cells e.g. bacteria and viruses (‘non-self’).
Each cell carries protein markers called antigens that allow it to be identified as ‘self’ or ‘non-self’ by the immune system. Antibodies (also known as immunoglobulins) are important blood proteins in the immune system that are produced by a type of white blood cell called B cells. Antibodies naturally develop during exposure to infection and by binding to foreign antigens, help neutralise and eliminate the organisms that caused the infection.
Antibodies are especially helpful when a particular type of bacteria or virus is encountered for a second time. In this situation antibodies already present in the blood against the particular bacteria or virus, prevent infection from developing. Antibodies, though, are just one aspect of immunity.
Our body also has another kind of white blood cells called the T cells. These are responsible for cell-mediated immunity. T-cell surfaces also have what is called histocompatibility antigens, which help us to accept or reject transplants of organs done to us.
Autoimmunity is a failure of the immune system to always correctly recognise ‘self’. As a result, the body’s own immune system attacks part of the human body. This leads to the formation of antibodies directed against one's own tissues - the autoantibodies. Autoantibodies may directly cause harm, by causing the body to malfunction.
There are many conditions with an underlying basis of autoimmunity. They include lupus, thyroid problems and many more.
(Ref: Mark McClure's article in www.vasculitis.org.uk)