Antibodies are an important part of the immune system. These proteins-also known as immunoglobulin's-help defend the body against foreign matter, especially bacteria, viruses and parasites. They are produced by B lymphocytes, white blood cells whose specific purpose is to make contact with foreign matter and then release one or more of a number antibodies into the blood. There are various types of antibodies, with immunoglobulin G occurring most commonly in blood. It has two bonding sites that can recognize an invading molecule, or antigen. When a bacterium carrying this antigen on its surface enters the body, the antibodies recognize and bind to the antigen so that the intruder is either paralyzed or destroyed by immune cells.
Immunisation against a specific disease prompts the immune system to produce immune cells and antibodies in the blood which, if the body becomes infected with the disease at some time in the futere, will try to destroy the pathogens.