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What is the Immune System?

Immunisation is one of the most important tools in preventive medicine. Humans have immune system that helps them to resist disease; nevertheless, there are some diseases that present a serious risk to life. For some of these diseases (caused by bacteria or virus) people can be made immune by artificial means. This is called immunisation. The main principle behind immunisation is to ensure that antibodies, to disease- causing organisms are present in the blood before exposure to the disease. If this is achieved, people will be unaffected, or only mildly affected.
The immune system is the collective army of a trillion white blood cells, bone marrow, antibodies, cytokines and the thymus gland that help to identify and destroy the millions of microbes (i.e. bacteria, viruses, parasites, fungi) that penetrate our bodies every day, and the thousands of our own cells that have become genetically abnormal or cancerous. Key to immune function is the activity of the white blood cells, such as NK, T and B cells, which form the backbone of the immune system
Immunity is the protection against disease provided by the body's defence system or immune system. May be obtained actively or passively. Immune response is the action of lymphocytes in response to the entry of an antigen to the body.
The body has many defences against pathogens entering e.g. skin, HCl in the stomach, ciliated epithelium in airways, blood clotting and white blood cells. An antigen is any molecule recognised by the body as foreign. They cover surface of pathogens and may be proteins, lipids, glycoproteins etc…
An antibody acts against a particular antigen. It is made of globular glycoproteins and produced by B lymphocytes (part of the immune response). Antigens / antibodies are specific. The immune system depends on two types of white blood cells: phagocytes and lymphocytes
How does Immune System strength determine health?